Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-03-03
2001-03-27
Young, Lee (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S831000, C029S848000, C219S457100
Reexamination Certificate
active
06205648
ABSTRACT:
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 Such microporous materials include aerogel, which is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid the shrinkage which would occur if the gel were dried directly from a liquid.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 filame
Ceramaspeed Limited
Dorman Ira S.
Smith Sean
Young Lee
LandOfFree
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