Abstrict
An electric heater unit is manufactured so as to form a base in
a supporting dish by compacting powdered microporous insulation
into the base. At least one electrical resistance heating element
is supported on or adjacent to the base and a peripheral wall is
formed in the supporting dish and integral with the base by compacting
further microporous insulation material into the dish to a controlled
compaction density. The compaction density of the peripheral wall
may be different from that of the base, for example higher.
Claims
What is claimed is:
1. A method of manufacturing an electric heater comprising the
steps of providing a supporting dish, forming in the supporting
dish a base by compacting powdered microporous insulation material
therein, providing at least one electrical resistance heating element
supported relative to the base, and forming in the supporting dish
a peripheral wall integral with the base, and having a density higher
than that of the base, by compacting further microporous insulation
material into the dish to a controlled density higher than the density
of the base.
2. A method according to claim 1, wherein a press tool is provided
having separable central and surrounding peripheral portions, powdered
microporous insulation material is compacted into the supporting
dish with the press tool to form the base, the peripheral portion
of the press tool is retracted to form a cavity into which further
powdered microporous insulation material is introduced, the peripheral
portion of the press tool is advanced to compact the further powdered
microporous insulation material to a controlled compaction density
to form the peripheral wall integral with the base, and the central
and peripheral portions of the press tool are retracted from the
dish.
3. A method according to claim 2, wherein powdered microporous
insulation material is compacted into the supporting dish with the
press tool to form part of the peripheral wall simultaneously with
the base.
4. A method according to claim 2, wherein the powdered microporous
insulation material is introduced into the press tool by way of
a tube through a wall thereof.
5. A method according to claim 4, wherein the powdered material
is pumped through the tube.
6. A method according to claim 5, wherein the material is pumped
by means selected from high pressure gas induction, a vane pump,
a diaphragm pump and a peristaltic pump.
7. A method according to claim 2, wherein the supporting dish is
circular and the press tool with separable central and surrounding
peripheral portions has a circular central portion and an annular
surrounding peripheral portion.
8. A method according to claim 1, wherein powdered microporous
insulation material is compacted into the supporting dish with a
press tool to form the base, the press tool and the dish are then
separated and a further press tool having separable central and
surrounding peripheral portions is provided, at least one electrical
resistance heating element is supported at a face of the central
portion of the press tool and is pressed by the press tool into
the surface of the base of compacted microporous insulation material
in the supporting dish for partial embedding therein, the peripheral
portion of the further press tool is retracted to form a cavity
into which further powdered microporous insulation material is introduced,
the peripheral portion of the further press tool is advanced to
compact the further powdered microporous insulation material to
a controlled compaction density to form the peripheral wall integral
with the base, and the central and peripheral portions of the further
press tool are retracted from the dish, leaving the heating element
securely partially embedded in the base.
9. A method according to claim 8, wherein powdered microporous
insulation material is compacted into the supporting dish with the
press tool to form part of the peripheral wall simultaneously with
the base.
10. A method according to claim 8, wherein the powdered microporous
insulation material is introduced into the press tool by way of
a tube through a wall thereof.
11. A method according to claim 10, wherein the powdered material
is pumped through the tube.
12. A method according to claim 11, wherein the material is pumped
by means selected from high pressure gas induction, a vane pump,
a diaphragm pump and a peristaltic pump.
13. A method according to claim 8, wherein the supporting dish
is circular and the press tool with separable central and surrounding
peripheral portions has a circular central portion and an annular
surrounding peripheral portion.
14. A method according to claim 1, wherein the further microporous
insulation material has a composition substantially the same as
that of the material forming the base.
15. A method according to claim 1, wherein the further microporous
insulation material has a composition different from that of the
material forming the base.
16. A method according to claim 1, wherein the peripheral wall
of compacted microporous insulation material is under internal compressive
strain after provision in the supporting dish.
17. A method according to claim 1, wherein the peripheral wall
is arranged to have a top surface at least part of which is capable
of contacting the underside of a glass ceramic cook top of a cooking
appliance.
18. A method according to claim 17, wherein the peripheral wall
has a height at least as great as the height of side walls of the
supporting dish.
19. A method according to claim 12, wherein the top surface of
the peripheral wall is profiled such that it is higher at its centre
than at its edges.
20. A method according to claim 1, wherein the peripheral wall
includes reinforcing glass filaments.
21. A method according to claim 20, wherein the reinforcing glass
filaments are selected from E glass, R glass, S glass and silica.
22. A method according to claim 1, wherein the base includes reinforcing
glass filaments.
23. A method according to claim 22, wherein the reinforcing glass
filaments are selected from E glass, R glass, S glass and silica.
24. A method according to claim 1, wherein the supporting dish
comprises a metal.
25. A method according to claim 1, wherein the at least one electrical
resistance heating element is selected from coiled wire, coiled
ribbon, and plane or corrugated ribbon, and is disposed flat or
edgewise relative to the base in the supporting dish.
26. A method of manufacturing an electric heater comprising the
steps: providing a supporting dish contained in a forming space;
introducing into the forming space a powdered microporous insulation
material; forming in the supporting dish a base by compacting the
powdered microporous insulation material therein; providing at least
one electrical resistance heating element supported relative to
the base; introducing into the forming space further microporous
insulation material; and forming in the supporting dish a peripheral
wall integral with the base, and having a density higher than that
of the base, at least in part by compacting the further microporous
insulation material into the dish to a controlled density higher
than the density of the base.
Description This invention relates to an electric heater unit, particularly
but not exclusively for use in glass-ceramic cooking appliances,
and a method for manufacture thereof.
DESCRIPTION OF PRIOR ART
Heaters for use in glass-ceramic surface electric cooking equipment
are well known, having an insulation material in the shape of a
bowl comprising a base and peripheral wall, the base supporting,
or having adjacent thereto, one or more heating conductors in the
form of an electrical resistance material formed as a wire coil,
a ribbon, a halogen infra red tube or other means.
The electrical and thermal insulation material is a critical component.
At least a part of the base insulation may be a high performance
insulation which is a compacted microporous material.
The term `microporous` is used herein to identify porous or cellular
materials in which the ultimate size of the cells or voids is less
than the mean free path of an air molecule at NTP, i.e. of the order
of 100 nm or smaller. A material which is microporous in this sense
will exhibit very low transfer of heat by air conduction (that is,
due to collisions between air molecules). Such microporous materials
include aerogel, which is a gel in which the liquid phase has been
replaced by a gaseous phase in such a way as to avoid the shrinkage
which would occur if the gel were dried directly from a liquid.
A substantially identical structure can be obtained by controlled
precipitation from solution, the temperature and pH being controlled
during precipitation to obtain an open lattice precipitate. Other
equivalent open lattice structures include pyrogenic (fumed) and
electro-thermal types in which a substantial proportion of the particles
have an ultimate size less than 100 nm. Any of these materials,
based, for example on silica, alumina, other metal oxides, or carbon,
may be used to prepare a composition which is microporous as defined
above.
Optionally a binder may be added to provide increased strength,
in which case a heat treatment may be necessary in order to cure
the binder.
A known form of high performance microporous thermal insulation
material comprises microporous silica particles compacted to consolidate
the material into a handleable form, and typically includes ceramic
fibre or glass filament reinforcement and rutile powder opacifier.
The microporous insulation may be directly in contact with the
heating conductor, acting as a support for the conductor.
Alternatively the conductor may be supported by a lesser thermal
insulation material which has mechanical properties quite different
from the microporous thermal insulation. In this case the base support
and peripheral wall may be formed as one piece with the wall and
base being a homogeneous material.
When the base is a microporous insulation it has been found to
be advantageous to have the peripheral wall made from a separate
stronger material. Heaters have been made which have wall and base
support formed as pressed microporous insulation material but the
walls were mechanically weak and a stronger material was fitted
to the top of the peripheral wall to improve handle ability.
Another design idea uses a microporous base support with a separate
wall component also made from microporous insulation. It is claimed
that the separate wall component can be made with high mechanical
strength and good insulation properties. The higher strength is
achieved by a special hardening process. This solution is costly.
The wall component is slow to produce and needs care in handling.
OBJECT OF THE INVENTION
It is an object of the present invention to provide a high strength
microporous wall component at low cost.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided
an electric heater unit comprising a supporting dish having therein
a base of compacted microporous insulation material, at least one
electrical resistance heating element supported relative (that is,
on or adjacent) to the base, and a peripheral wall of compacted
microporous insulation material, wherein the peripheral wall is
integral with the base and is of controlled compaction density.
According to a further aspect of the invention there is provided
a method of manufacturing an electric heater unit comprising the
steps of providing a supporting dish, forming in the supporting
dish a base by compacting powdered microporous insulation material
therein, providing at least one electrical resistance heating element
supported relative (that is, on or adjacent) to the base, and forming
in the supporting dish a peripheral wall integral with the base
by compacting further microporous insulation material into the dish
to a controlled compaction density.
The compaction density of the peripheral wall may be different
from that of the base. For example, the peripheral wall may be of
higher compaction density than the base.
In one embodiment of the method according to the invention a press
tool is provided having separable central and surrounding peripheral
portions, powdered microporous insulation material is compacted
into the supporting dish with the press tool to form the base and,
optionally, part of the peripheral wall, the peripheral portion
of the press tool is retracted to form a cavity into which further
powdered microporous insulation material is introduced, the peripheral
portion of the press tool is advanced to compact the further powdered
microporous insulation material to a controlled compaction density
to form the peripheral wall integral with the base, and the central
and peripheral portions of the press tool are retracted from the
dish.
In a further embodiment of the method according to the invention,
powdered microporous insulation material is compacted into the supporting
dish with a press tool to form the base and, optionally, part of
the peripheral wall, the press tool and the dish are then separated
and a further press tool having separable central and surrounding
peripheral portions is provided, at least one electrical resistance
heating element is supported at a face of the central portion of
the press tool and is pressed by the press tool into the surface
of the base of compacted microporous insulation material in the
supporting dish for partial embedding therein, the peripheral portion
of the further press tool is retracted to form a cavity into which
further powdered microporous insulation material is introduced,
the peripheral portion of the further press tool is advanced to
compact the further powdered microporous insulation material to
a controlled compaction density to form the peripheral wall integral
with the base, and the central and peripheral portions of the further
press tool are retracted from the dish, leaving the heating element
securely partially embedded in the base.
The powdered microporous insulation material (including, optionally,
the further powdered microporous insulation material) may be introduced
into the press tool by way of a tube through a wall thereof. The
powdered material may be pumped through the tube such as by using
high pressure gas induction or by using a vane pump, a diaphragm
pump or a peristaltic pump.
The supporting dish may be circular and the press tool with separable
central and surrounding peripheral portions may have a circular
central portion and an annular surrounding peripheral portion.
The further powdered microporous insulation material may have a
composition substantially the same as, or different from, that of
the material forming the base.
The peripheral wall of compacted microporous insulation material
preferably is under internal compressive strain after provision
in the supporting dish.
The peripheral wall is suitably arranged to have a top surface
capable of contacting the underside of a glass-ceramic cook top
of a cooking appliance, in particular the peripheral wall may have
a height at least as great as the height of side walls of the supporting
dish. Such top surface may be profiled such that it is higher at
its centre than at its edges.
The peripheral wall and/or the base may include reinforcing glass
filaments. Such filaments may, for example, be selected from E glass,
R glass, S glass and silica.
The supporting dish may comprise a metal.
The at least one electrical resistance heating element may, for
example, comprise coiled wire or coiled ribbon, or plane or corrugated
ribbon, disposed flat or edgewise relative (that is, on or adjacent)
to the base in the supporting dish.
By means of the invention a peripheral wall of microporous insulation
material is provided in which the composition and the compaction
density thereof are the same as or different from a base of microporous
insulation material with which it is integrally provided.
The invention is now described by way of example with reference
to the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 to 7 are cross-sectional views of an arrangement illustrating
process steps in the manufacture of an electric heater unit according
to the invention;
FIGS. 8 to 15 are cross-sectional views of a further arrangement
illustrating process steps in the manufacture of an electric heater
unit according to the invention; and
FIG. 16 is a plan view of an embodiment of electric heater unit
manufactured according to the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 1, a press for use in manufacturing an electric
heater unit according to the invention comprises a housing 1, a
cover 2 and a press tool 3 which is slidable inside the housing
1. The end of the housing 1 is recessed to receive the rim of a
metal dish 4 which will form the supporting dish for the electric
heater unit.
The press tool 3 is of circular shape and comprises a circular
central portion 3A and an annular surrounding peripheral portion
3B. The central portion 3A and annular portion 3B are separable
from one another and are slidable in the housing by means of plungers
5A and 5B.
The central portion 3A has an extended cylindrical wall 6 able
to slidably interface with the annular portion 3B.
Operation of the press commences with retraction of the press tool
3 to the position shown in FIG. 1.
A predetermined quantity of powdered microporous thermal insulation
material is introduced into the space 7 between the press tool 3
and the dish 4. By way of example only, the insulation material
may have the following composition:
Pyrogenic silica 60 percent by weight Opacifier (Rutile) 37 percent
by weight Ceramic fibres 3 percent by weight
The powdered material may be introduced into the space 7 before
the dish 4 and cover 2 are installed. Alternatively it may be pumped
into the space 7 by way of a tube T passing through the wall of
the housing 1. Pumping of the powder through the tube T may be by
using high pressure gas induction or using a vane pump, a diaphragm
pump, or a peristaltic pump.
The press is operated, for example hydraulically, to urge both
portions 3A and 3B of the press tool simultaneously towards the
dish 4, by means of the plungers 5A and 5B, as shown in FIG. 2,
thereby compacting the insulation material into the dish 4 to form
a base 8 in the dish.
During the compacting operation, air is displaced from the press
through holes 9 at the periphery of the dish 4 and also via the
interface between the press tool 3 and the housing 1. If required,
holes (not shown) may be provided through the press tool 3 to further
facilitate air displacement.
The compacted insulation material forming the base 8 may be formed
with a step 8A at the edge thereof, by forming a complementary step
in the press tool 3. Such step 8A forms a base portion of a peripheral
wall of insulation material which is to be provided in the dish
as hereinafter described.
As shown in FIG. 3, the next step in the process is to retract
the annular portion 3B of the press tool, by means of the plungers
5B while leaving the central portion 3A of the press tool in contact
with the surface of the base 8 of insulation material. Further powdered
microporous insulation material is then pumped through the tube
T into the space 10 vacated by the annular portion 3B of the press
tool.
As shown in FIG. 4, the annular portion 3B of the press tool is
then advanced towards the dish 4 to compact the further insulation
material to form a peripheral wall 11 of microporous insulation
material integrally moulded with the base 8 of microporous insulation
material. The wall 11 is arranged to be compacted to a higher compaction
density than that on average of the base 8. For example the base
8 may have a compaction density of about 300 kg/m.sup.3 whereas
the wall 11 may be compacted to a density of about 350 kg/m.sup.3.
The wall 11 may have a composition the same as, or different from,
that of the base 8. An example of a particular composition for the
wall is:
Pyrogenic silica 62 percent by weight Opacifier (Rutile) 27 percent
by weight E glass filaments 11 percent by weight
Both portions 3A, 3B of the press tool are then retracted as shown
in FIG. 5, the cover 2 is removed and the dish 4 with the base 8
and peripheral wall 11 therein is extracted. The dish 4 with the
base 8 and peripheral wall 11 therein is shown in FIG. 6. The peripheral
wall 11 has a height corresponding at least to the height of side
walls of the dish 4, and preferably extending somewhat above the
side walls of the dish 4.
To complete the heater unit, an electrical resistance heating element
12 is provided supported on the base 8 of microporous thermal insulation
material as shown in FIG. 7. Heating element 12 may comprise any
of the well known forms, such as coiled wire or ribbon or a corrugated
ribbon supported edgewise and partly embedded in the base 8. Such
a corrugated ribbon form of element is shown in FIG. 7 and also
in FIG. 16, which represents a plan view of the heater of FIG. 7
and in which there is additionally provided a well known form of
temperature limiter 13.
The heater of FIGS. 7 and 16 is intended for operation in a glass-ceramic
top cooking appliance (not shown) where it is secured beneath a
glass-ceramic cook top (not shown) with the upper surface 11A of
the peripheral wall in contact with the underside of the glass-ceramic
top.
As shown by the dotted outline 14 in FIGS. 6 and 7, the top surface
of the peripheral wall 11 may be profiled such that it is higher
at its centre than at its edges. This is achieved by providing a
complementary profile on the inner face 14A of the annular portion
3B of the press tool (FIG. 5).
FIGS. 8 to 15 illustrate an alternative process sequence including
the moulding of corrugated ribbon heating element 12 into base 8
of microporous insulation material.
Referring to FIG. 8, a press is provided, as in FIG. 1, comprising
a housing 1 recessed to receive the rim of a metal dish 4 forming
the supporting dish of an electric heater unit. A cover 2 is provided
for the housing. A circular press tool 3 is provided, slidable in
the housing 1 by means of a plunger 5.
With the press tool in the position shown in FIG. 8, a predetermined
quantity of powdered microporous thermal insulation material is
introduced into the space 7 between the press tool 3 and the dish
4 using either of the methods as previously described with reference
to FIG. 1. The press is operated to urge the press tool 3 towards
the dish 4, as shown in FIG. 9, thereby compacting the insulation
material into the dish 4 to form a base 8 in the dish. The press
tool 3 is then withdrawn from the housing and replaced by the press
tool shown in FIG. 10, which is of two part form as previously described
with reference to FIG. 1, having a central circular portion 3A operated
by a plunger 5A and an annular peripheral portion 3B operated by
plungers 5B. The top surface 15 of the central portion 3A of the
press tool is provided with a pattern of grooves to partially receive
therein a corrugated ribbon heating element 12. The press tool 3A,
3B is advanced by means of plungers 5A, 5B towards the base 8 of
compacted insulation material, as shown in FIG. 11, to cause the
heating element 12 to be partially embedded in the surface of the
base 8. It may be advantageous if, during the initial provision
of the base 8, as described with reference to FIGS. 8 and 9, the
microporous insulation material of the base 8 is compacted to less
than its required final density. This facilitates embedding of the
heating element 12 therein and subsequent to such embedding, the
base 8 is compacted to its desired final density by pressure exerted
thereon by the surface of the press tool 3A, 3B.
With the central portion 3A of the press tool retained in the position
shown in FIG. 11, the annular peripheral portion 3B of the press
tool is retracted by the plungers 5B into the position shown in
FIG. 12. Further powdered microporous insulation material is then
pumped through tube T into the space 10 vacated by the annular portion
3B of the press tool.
As shown in FIG. 13, the annular portion 3B of the press tool is
then advanced towards the dish 4 to compact the further insulation
material to form a peripheral wall 11 of microporous insulation
material integrally moulded with the base 8 of microporous insulation
material. The wall 11 is arranged to be compacted to a higher compaction
density than that on average of the base 8.
The wall 11 may have a composition the same as, or different from,
that of the base 8.
Both portions 3A, 3B of the press tool are then retracted as shown
in FIG. 14, leaving the heating element 12 securely partially embedded
in the base 8. It is preferred that the central portion 3A of the
press tool is retracted before the annular portion 3B to minimise
risk of damage to material of the wall 11. The cover 2 is removed
from the press and the heater unit comprising the dish 4, with the
base 8, peripheral wall 11 and heating element 12, extracted. Such
heater unit is shown in section in FIG. 15 and, after the addition
of a temperature limiter 13, in plan view in FIG. 16.
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