Abstrict
An electrical heating element for a radiant electric heater takes
the form of an elongate electrically conductive strip for partial
embedding edgewise in a base of thermal and electrical insulation
material. The strip is composed of an elongate continuous portion
and an elongate discontinuous portion integral with the continuous
portion. The discontinuous portion is intended for embedding in
the base and is provided with a plurality of discontinuities so
that in operation of the heater current flow in the discontinuous
portion is reduced or eliminated.
Claims
I claim:
1. A method of manufacturing a radiant electric heater comprising:
providing a base of compacted microporous thermal and electrical
insulation material;
providing a heating element in the form of an elongate electrically
conductive strip, the strip being of uniform thickness and composed
of an elongate continuous portion and an elongate discontinuous
portion integral and coplanar with the continuous portion, the discontinuous
portion being provided with a plurality of discontinuities therein
such that in operation of the heater current flow in the discontinuous
portion is reduced or eliminated; and
pressing the heating element edgewise into the base so as to at
least partially embed the discontinuous portion of the heating element
in the base with the insulation material entering the discontinuities
to thereby enhance securement of the heating element to the base.
2. A method according to claim 1, wherein discontinuities are provided
extending from the leading edge of the strip, which is embedded.
3. A method according to claim 1, wherein the area of the discontinuities
of the discontinuous portion is less than the remainder of the area
of the discontinuous portion.
4. A method according to claim 3, wherein the discontinuities comprise
slits the discontinuous portion of the strip.
5. A method according to claim 1, wherein the discontinuities are
in spaced relationship along the length of the discontinuous portion
of the strip.
6. A method according to claim 1, wherein the discontinuities are
straight in the discontinuous portion of the strip.
7. A method according to claim 1, wherein the discontinuities are
curved in the discontinuous portion of the strip.
8. A method according to claim 1, wherein the discontinuities are
angled in the discontinuous portion of the strip.
9. A method according to claim 1, wherein the discontinuities are
uniform.
10. A method according to claim 1, wherein the discontinuities
are non-uniform.
11. A method according to claim 1, wherein the discontinuities
extend at least up to a level to which the strip is embedded.
12. A method according to claim 2 wherein the area of the discontinuities
of the discontinuous portion is substantially equal to the remainder
of the area of the discontinuous portion.
13. A method according to claim 1, wherein the discontinuities
comprise slots the discontinuous portion of the strip.
14. A method according to claim 1, wherein the discontinuities
comprise holes formed in the discontinuous portion of the strip.
15. A method according to claim 14, wherein the holes are dimensioned
such that they span a substantial part of that depth of the strip
which is embedded.
16. A method according to claim 15, wherein the holes extend at
least up to a level to which the strip is embedded.
17. A method according to claim 1, wherein the strip is of corrugated
form along its length.
18. A method according to claim 1, wherein the material of the
strip is selected from the group consisting of a metal and a metal
alloy.
Description This invention relates to an electrical heating element for a radiant
electric heater, more particularly, but not exclusively, for use
with a glass-ceramic smooth top cooker. More particularly, the invention
relates to an electrical heating element in the form of an elongate
strip of electrically conductive material, such as a metal or a
metal alloy, for partial embedding edgewise into a base of thermal
and electrical insulation material, such as microporous thermal
and electrical insulation material. The invention also relates to
a radiant electric heater incorporating such an electrical heating
element.
BACKGROUND TO THE INVENTION
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
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
particle size less than 100 nm. Any of these particulate materials,
based for example on silica, alumina or other metal oxides, may
be used to prepare a composition which is microporous as defined
above.
The microporous insulation typically comprises a dry particulate
microporous material as defined hereinabove mixed with ceramic fibre
reinforcement, titanium dioxide opacifier and, for high-temperature
use, a small quantity of alumina powder to resist shrinkage. Such
microporous insulation material is described in GB-A-1 580 909.
When a heating element in the form of an elongate strip of electrically
conductive material is partially embedded edgewise into a base of
insulation material, such as microporous thermal and electrical
insulation material, to form a radiant heater, during operation
of the heater the strip reaches a high temperature required for
satisfactory operation of the heater. However, the entire strip
comprising exposed and embedded regions operates at a high temperature
and it would be advantageous if the embedded portion of the strip
could be arranged to attain a lower temperature than the exposed
region when the heater is operated.
It is known from GB-A-1 569 588 to provide a heating conductor
strip which is slotted alternately from opposite edges and is provided
with spaced anchoring tabs which extend from the strip and penetrate
an underlying insulating sheet. As acknowledged in GB-A-1 569 588
the slots have the disadvantageous effect of reducing the rigidity
of the heating conductor strip in all directions. Moreover, the
anchoring tabs are spaced at a distance of several slots from one
another. The overall effect of the low rigidity heating conductor
and the relatively wide spacing of the anchoring tabs gives rise
to undesirable distortion of the heating conductor as a result of
the regular cycles of heating and cooling to which the heating conductor
is subjected. This in turn can lead to adjacent turns of the heating
conductor becoming too close to each other and giving rise to an
electrical short circuit or to the heating conductor coming into
contact with a temperature sensor which is conventionally provided
in radiant heaters for glass-ceramic smooth top cookers.
U.S. Pat. No. 4,292,504 describes an electric resistance heating
unit in which the heating element comprises a thin, foil-like strip
of expanded metal supported on edge substantially along its entire
length on a board of insulating material. The heating element is
either cemented or a close fit within a groove formed in the board.
Such an expanded metal heating element occupies an undesirable amount
of space within the heater and is relatively fragile: it additionally
gives rise to the same flow of electric current in the portion of
the heating element within the groove as in the exposed portion
thereof and the embedded portion of the heating element therefore
rises to an undesirably high temperature.
U.S. Pat. No. 600,057 describes an electric heater in which a conductor
stamped into one of a number of shapes is attached edgewise to a
support by one or more layers of enamel. The conductor is shaped
in order to prevent damage to the enamel which is used to attach
the conductor to the support. By way of example, the conductor may
be provided with a series of lateral projections, which may be of
any desired shape, which projections are partially or entirely embedded
in the enamel, with the body of the conductor being exposed.
OBJECT OF THE INVENTION
It is an object of the present invention to provide a heating element
in the form of an elongate strip of electrically conductive material
in which one elongate portion of the strip is able to operate at
a lower temperature than another portion thereof. Such a heating
element should give rise to a more rapid heating up of the element
to radiance.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided
an electrical heating element for a radiant electric heater, the
element comprising an elongate electrically conductive strip for
partial embedding edgewise in a base of thermal and electrical insulation
material, the strip being composed of an elongate continuous portion
and an elongate discontinuous portion integral with the continuous
portion, the discontinuous portion being intended for embedding
in the base and being provided with a plurality of discontinuities
therein, whereby in operation of the heater current flow in the
discontinuous portion is reduced or eliminated.
According to another aspect of the present invention there is provided
a radiant electric heater comprising a base of thermal and electrical
insulation material and an elongate electrically conductive strip
partially embedded edgewise therein, the strip being composed of
an elongate continuous portion and an elongate discontinuous portion
integral with the continuous portion, the discontinuous portion
being at least partially embedded in the base and being provided
with a plurality of discontinuities therein, whereby in operation
of the heater current flow in the discontinuous portion is reduced
or eliminated.
The term "continuous" is used herein in respect of the
continuous portion of the strip to denote an elongate portion of
the strip which is of substantially uniform dimensions. In contrast,
the term "discontinuous" is used herein in respect of
the discontinuous portion of the strip to denote a portion of the
strip, adjacent to the continuous portion, which is provided with
tabs or the like which extend in a direction away from the continuous
portion such that the cross-sectional area of material, in a direction
substantially perpendicular to the elongate direction of the continuous
portion of the strip, is variable in the elongate direction of the
strip. Thus, at some points in the elongate direction of the strip
the cross-sectional area of material may be substantially reduced
as compared with other points, or there may be no material at all.
The discontinuities serve a number of purposes. Firstly, during
operation of the element in a heater they reduce or eliminate electric
current flow and hence self-heating of the strip along the length
of the strip in that region thereof where they are provided and
a lower temperature results in the strip in this region, which is
embedded in the insulation material, compared with the remaining
exposed region of the strip. Secondly, the discontinuities serve
to enhance securement of the strip to the insulation material when
embedded therein. Thirdly, the discontinuities reduce the mass of
the strip and this leads to more rapid heating up to radiance of
the exposed region of the strip. Fourthly, the discontinuities can
be arranged such that they reduce thermal conduction from the region
of the strip which is to be exposed, into that region which is to
be embedded.
The discontinuous portion of the strip may be coplanar with the
continuous portion thereof.
The discontinuities should ideally be provided as close together
as is reasonably possible along the length of the strip. The area
of the discontinuities of the discontinuous portion in the plane
thereof may be less than the remainder of the area of the discontinuous
portion. The discontinuities may be in spaced relationship along
the length of the discontinuous portion of the strip.
The discontinuities may comprise slots or slits formed in the discontinuous
portion of the strip. The slots or slits may be provided extending
from that edge of the discontinuous portion of the strip which is
to be embedded. The slots or slits may be straight or bent or curved,
and/or angled in the discontinuous portion of the strip. The slots
or slits may be uniform or non-uniform. The slots or slits may extend
up to or beyond a level to which the strip is to be embedded.
Alternatively, the discontinuities may comprise holes formed in
the discontinuous portion of the strip. Holes of a required shape,
such as rectangular, circular or oval, may be provided in the discontinuous
portion of the strip, the holes being dimensioned such that they
span a substantial part of that depth of the strip which is to be
embedded. The holes may extend up to or beyond a level to which
the strip is to be embedded.
The strip may be of corrugated (sometimes also known as sinuous
or serpentine or convoluted) form along its length.
The strip suitably comprises a metal or a metal alloy, such as,
for example, an iron-chromium-aluminium alloy. However the invention
is not limited to any particular material or configuration of the
strip.
The insulation material may comprise microporous thermal and electrical
insulation material.
Suitable microporous thermal and electrical insulation materials
are well-known in the art, for example as described in GB-A-1 580
909, a typical composition being:
______________________________________ Microporous pyrogenic silica
49 to 97% by weight Ceramic fibre reinforcement 0.5 to 20% by weight
Opacifier 2 to 50% by weight Alumina up to 12% by weight ______________________________________
The proportion of alumina is preferably in the range from 0.5 to
12 percent by weight.
The invention is now described by way of example with reference
to the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view of a first embodiment of an electrical
heating element according to the present invention;
FIG. 2 is a side elevational view of a second embodiment of an
electrical heating element according to the present invention;
FIG. 3 is a side elevational view of a third embodiment of an electrical
heating element according to the present invention;
FIG. 4 is a side elevational view of a fourth embodiment of an
electrical heating element according to the present invention; and
FIG. 5 is a side elevational view of a fifth embodiment of an electrical
heating element according to the present invention;
FIG. 6 is a side elevational view of a sixth embodiment of an electrical
heating element according to the present invention;
FIG. 7 is a side elevational view of a seventh embodiment of an
electrical heating element according to the present invention; and
FIG. 8 is a plan view of a radiant electric heater incorporating
an electrical heating element according to the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 1, an electrical heating element for a radiant
electric heater consists of an elongate electrically conductive
strip 1, of a metal or a metal alloy, which is intended to be embedded
edgewise, to a level substantially represented by dashed line 2,
in a base of electrical and thermal insulation material 3. Such
base may, for example, comprise compacted microporous thermal and
electrical insulation material such as that described in GB-A-1
580 909.
The strip 1 comprises a continuous portion (the upper portion as
illustrated in FIG. 1) and a discontinuous portion (the lower portion
as illustrated in FIG. 1). The lower, discontinuous portion is provided
with a plurality of discontinuities in the form of spaced-apart
slits 4 extending into the strip 1, from the edge thereof and substantially
up to the level 2 to which the strip is intended to be embedded
if the strip 1 is to be embedded to the top of the slits 4 or up
to the level 2' to which the strip is to be embedded if the strip
1 is to be embedded to a level below the top of the slits 4. The
slits 4 should ideally be disposed as close together as is reasonably
possible. As illustrated in FIG. 1, the slits 4 occupy a minor proportion
of the area of the discontinuous portion of the strip 1.
If the strip when embedded is connected at its ends to a suitable
voltage source, for operation is a heating element in a radiant
heater, electric current will flow through the exposed, that is
unembedded, continuous portion of the strip 1 as shown by the arrow
5, resulting in the desired heating of the strip in this region
to fulfil its function as a heating element. In the at least partially
embedded discontinuous portion of the strip, however, because of
the presence of the slits 4, little or no electric current can flow
along where indicated by the arrow 6. This means that substantially
no electrical heating of the discontinuous portion of the strip
1 occurs. Consequently, the temperature attained by the strip in
the discontinuous portion is very much lower than that attained
by the strip in the continuous portion. This is advantageous in
that it prevents undesirable high temperatures being attained in
the region of the insulation material.
The arrangements shown in FIG. 2 to 7 operate in a similar manner
to the arrangement of FIG. 1.
In FIG. 2, instead of the slits 4 of FIG. 1, slots 4A are provided,
likewise disposed relatively close together and extending into the
discontinuous portion of the strip 1 from the edge thereof and substantially
up to the level 2 or 2' to which the strip 1 is intended to be embedded
in the insulation 3. As illustrated in FIG. 2, the slits 4A occupy
only about half the area of the discontinuous portion of the strip
1.
In FIGS. 3 and 4, rows of circular holes 4B and square holes 4C
respectively are provided which are dimensioned such that they span
as much as possible of the distance between the edge of the strip
which is to be embedded and the level 2 or 2' to which it is to
be embedded. The slots 4A in FIG. 2 and the holes 4B and 4C in FIGS.
3 and 4 fulfil the same role as described for the slits 4 in FIG.
1, namely in minimizing flow of electric current along the strip
1 in the embedded region thereof when the strip is operating as
a heating element.
In FIG. 5, the slits 4 are bent or curved, in FIG. 6, the slits
4 are angled, and in FIG. 7 the slits 4 are non-uniform along their
length.
If desired, the slits 4, slots 4A and holes 4B, 4C may be arranged
so that they extend beyond the level to which the strip is to be
embedded in the insulation material 3 as shown by the level 2'.
With such an arrangement, the slits 4, slots 4A and holes 4B, 4C
will be partially exposed when the strip 1 is embedded in the insulation
material 3.
The slits 4, slots 4A and holes 4B, 4C fulfil a further function
in that when the strip 1 is embedded, for example by pressing edgewise
into the insulation material 3, insulation material enters the slits,
slots or holes and this assists in securing the strip 1 in its embedment
in the insulation material 3. This is particularly advantageous
when the insulation material is microporous insulation material.
I have found that microporous thermal insulation material, even
when compacted into a metal dish, retains a certain amount of resilience
and this assists in retaining the strip 1 in the base due to friction.
Further advantages resulting from the use of the slits, slots or
holes are as follows. They reduce the mass of the strip 1 and this
leads to more rapid heating up of the continuous portion of the
strip to a radiant operating temperature. Such rapid heating up
to radiance is also enhanced by the slots, slits or holes serving
as a means to reduce thermal conduction from the continuous portion
of the strip into the discontinuous portion which is embedded. In
this latter regard, it may be advantageous to provide slots or slits
which are bent or curved or angled with respect to one another in
the strip.
The strip 1 may, if desired, be provided of corrugated form along
its length.
FIG. 8 illustrates an application of a heating element according
to the present invention to a radiant electric heater for use under
a glass-ceramic plate of a smooth top cooker. A heating element
comprising an elongate electrically conductive strip 1 of a metal
or a metal alloy, such as an iron-chromium-aluminium alloy, constructed
as shown in any one of FIGS. 1 to 7 and described above with reference
thereto, is made into corrugated form and partially embedded by
pressing edgewise into the surface of a base layer 3 of microporous
thermal and electrical insulation material, such as that described
in GB-A-1 580 909, compacted into a metal dish 7. After embedding,
the surface of the base layer 3 is arranged to be substantially
at the level 2 or 2' shown in FIGS. 1 to 7. Against the side of
the dish 7 is located a peripheral wall 8 of thermal insulation
material such as ceramic fibre material or microporous insulation
material. A terminal connector 9 is provided for electrically connecting
the heating element strip 1 to an electrical supply.
A well-known form of thermal cut-out device 10 is provided extending
over the heating element 1 to switch off the heating element in
the event of over-heating of the glass-ceramic cooking surface when
the heater is installed and operating in a cooker provided with
such a glass-ceramic cooking surface.
|