GB2280578A

GB2280578A - Dual portion heating element

Info

Publication number: GB2280578A
Application number: GB9315589A
Authority: GB
Inventors: Kevin Ronald Mcwilliams
Original assignee: Ceramaspeed Ltd
Current assignee: Ceramaspeed Ltd
Priority date: 1993-07-28
Filing date: 1993-07-28
Publication date: 1995-02-01
Anticipated expiration: 2013-07-28
Also published as: EP0637194B1; DE69405834T2; GB9315589D0; GR3025466T3; GB2280578B; DK0637194T3; ATE158684T1; ES2107138T3; DE69405834D1; EP0637194A1

Abstract

A radiant electric heater, for example for use with a glass ceramic top cooker, comprises a support dish (1) containing a base layer (2) of thermal and electrical insulating material and at least one electrical heating element supported by the insulating material. The heating element comprises first and second portions electrically connected in series for operation, the first portion (4) comprising an elongate strip of resistive material and the second portion (5) comprising coiled bare resistance wire. <IMAGE>

Description

J 2280578 Radiant Electric Heater This invention relates to radiant

electric heaters, particularly for use with glass ceramic top cookers.

Radiant electric heaters are well known which comprise a metal dish, e.g. of circular, rectangular or oval shape, containing a base layer of thermal and electrical insulating material supporting one or more electrical heating elements. A peripheral wall of insulating material is also generally provided in the dish.

The most commonly-used form of heating element to date has comprised coiled bare resistance wire, for example as generally denoted by reference numeral 5 in Figure 1. With such wire, which may for example comprise an iron-chromiumaluminium alloy, it has been possible to select appropriate combinations of the wire diameter d, the coil diameter C, the total length of wire in the coil, and the pitch P of adjacent turns of the coil, to provide one or more heated zones in which the heating element produces a required power and heat distribution, with adequate operating life expectancy. Such wire coil elements are readily adaptable to heaters of different sizes and to requirements for different operating voltages by appropriate variation of the above parameters.

Ongoing trends in radiant heater design have been towards the achievement of heaters in which the heat up time to full radiance after switching on is as short as possible. In this regard it has been found advantageous to use, instead of a heating element in the form of coiled wire, an element comprising a thin elongate strip or ribbon of a metal or metal alloy. This is because the strip or ribbon element will have a smaller thermal mass than the equivalent element in the form of the coiled wire.

9 In order to design a practical heater, a strip or ribbon form as shown in Figure 2 is generally adopted. The strip or ribbon is of corrugated form (sometimes referred to as crinkled, or sinuous, or serpentine, or convoluted form) in order to accommodate the required length of ribbon within the heated zone. As with a heater designed using a coiled wire element, a number of parameters of the strip or ribbon can be varied in order to achieve the required heater performance. These parameters, as identified with reference to Figure 2, comprise: thickness t of the strip or ribbon; width w of the strip or ribbon; total length of the strip or ribbon; and corrugation pitch 10 x and corrugation depth y of the strip or ribbon.

When an operating voltage is applied, the strip or ribbon element heats up to full radiance much faster than a corresponding coiled wire element. In a typical arrangement the strip or ribbon element is found to heat up in about one third of the is time required by a corresponding coiled wire element.

A disadvantage has been found with strip or ribbon elements in that they are less versatile than coiled wire elements in meeting the requirements for heaters of different sizes and operating voltages. In spite of the facility for corrugating the strip or ribbon it has been found that,to achieve a predetermined end-to-end electrical resistance value for the element, a much longer strip or ribbon element is required than for a corresponding coiled wire element. In a particular example a strip or ribbon element may be required to be more than twice as long as an equivalent coiled wire element. This is because of practical limitations imposed on the minimum value of the corrugation pitch x and the maximum value of the corrugation depth y. It is therefore more demanding to accommodate a strip or ribbon element in a particular area of a heater than an equivalent coiled wire element and the problem becomes increasingly acute as the size of the area to be heated decreases and also as the voltage at which the heater is required to be operated increases. For example, in the case of a heater which is required to operated at 400 volts as compared with 230 volts, the length of the strip or ribbon must be correspondingly increased to increase the electrical resistance of the element in order to achieve the specified power. In such circumstances it may prove impossible to accommodate the required length of strip or ribbon in the allocated area of the heater.

It is an object of the present invention to overcome or minimise this problem.

The present invention provides a radiant electric heater comprising a support dish containing a base layer of thermal and electrical insulating material and at least one electrical heating element supported on the insulating material, the heating element comprising first and second portions electrically connected in series for operation; the first portion comprising an elongate strip or ribbon of a metal or a metal alloy supported on edge on the insulating material, the second portion comprising coiled bare resistance wire; the series combination of the first and second portions being adapted to be connected to a voltage source for operation of the heater.

The first portion of the element is preferably arranged to constitute the majority of the element.

The series connection between the first and second portions may be effected directly between ends thereof or by way of a connector mounted on the dish. Such a connector may also be used for connecting the element to a voltage source.

The first and second portions may conveniently comprise the same metal or metal alloy composition, e.g. nickel-chromium alloy or iron-chromium-aluminium alloy.

4The elongate strip or ribbon of the first portion is preferably corrugated in form.

The electrical heating element may be provided singly in a heater, or one or more such elements may be provided in a heater having multiple heated zones.

By means of the invention, the advantageous use of a strip or ribbon heating element may be maximised while making use of a proportion of coiled wire element to enable the available heater area, andlor supply voltage requirement, to be accommodated.

The invention is now described by way of example with reference to Figures 3 and 4 of the accompanying drawings in which:

Figure 3 represents a plan view of a radiant electric heater according to the is invention; and Figure 4 represents a perspective view of a strip or ribbon of a metal or metal alloy such as is used for a first portion of heater element in the heater of Figure 3.

A radiant electric heater for use under a glass ceramic top of a cooker is constructed comprising a metal dish 1 of well known form containing a base layer 2 of thermal and electrical insulating material. Such insulating material suitably comprises microporous thermal and electrical insulating material which is well known to the skilled person. A peripheral wall 3 of insulating material is provided, the top surface of which contacts the underside of the glass ceramic top of the cooker (not shown) when the heater is installed for operation.

A heating element is provided, supported on the base 2 and comprising a first portion 4 and a second portion 5. The first portion 4 constitutes the major part of the element and comprises an elongate strip or ribbon of a metal or metal alloy, such as an iron-chromium-aluminium alloy. The strip or ribbon portion 4 is provided of corrugated form, as indicated in detail by reference numeral 6, and is supported on edge. Figure 4 exemplifies a shaped strip or ribbon of the type used in portion 4 of Figure 3 although the actual pattern shown in Figure 4 is somewhat different from that of portion 4 in Figure 3. It would be preferred to provide the entire heating element of Figure 3 in the strip or ribbon form of portion 4, but as explained hereinbefore this is not possible with heaters of small heated zone areas and/or where a heater is required to be operated at a high voltage, e.g. 400 volts rather than, say, 230 volts. In order to achieve a required end to end electrical resistance value to provide an element of a desired power, a strip or ribbon element is required to be considerably longer than an element comprising a well known form of coiled bare resistance wire. Consequently there can be insufficient space within the heater to accommodate an element entirely of strip or ribbon form.

In the arrangement according to the invention as shown in Figure 3, as much as is conveniently possible of the element is provided by the strip or ribbon of portion 4 and the remainder of the element, to provide the required overall resistance value thereof, is made up by portion 5 which comprises coiled bare resistance wire of known form. Such coiled wire is suitably composed of a metal or metal alloy similar to the strip or ribbon of portion 4, e. g. iron-chromium-aluminium alloy. An example of coiled resistance wire was described earlier with reference to Figure 1. The portions 4 and 5 are electrically connected in series, suitably at a connector 7 which is also used for connecting the heater to a voltage supply. The portion 5 of coiled resistance wire may suitably form a complete turn around the periphery of the strip or ribbon portion 4 of the element. Depending upon the size of the heater and/or the operating voltage, one or more further turns of coiled resistance wire may be required for portion 5 surrounding the portion 4. However, in order to obtain maximum benefit in the heater from the use of the strip or ribbon portion 4, particularly with regard to the more rapid heat up time of the strip or ribbon compared with the coiled wire, the proportion of coiled wire (i.e. the proportion of portion 5 in the element) should be kept as small as possible.

The resulting heater may be provided with a well-known form of thermal limiter (not shown) extending across it and serving to prevent overheating when the heater is 10 installed and operating in a cooking appliance with a glass ceramic top.

It will be apparent to those skilled in the art that a number of alternative arrangements of the portions 4 and 5 can be envisaged. These could include the strip or ribbon portion 4 surrounding the coiled wire portion 5, or portions 4 and 5 interspersed with one another, whilst retaining the series connection between the portions.

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Claims

1. A radiant electric heater comprising a support dish containing a base layer of thermal and electrical insulating material and at least one electrical heating element supported on the insulating material, the heating element comprising first and second portions electrically connected in series for operation; the first portion comprising an elongate strip or ribbon of a metal or a metal alloy supported on edge on the insulating material, the second portion comprising coiled bare resistance wire; the series combination of the first and second 10 portions being adapted to be connected to a voltage source for operation of the heater.

2. A heater according to Claim 1, in which the first portion constitutes the majority of the element.

3. A heater according to Claim 1 or 2, in which series connection between the first and second portions is effected directly between ends thereof.

4. A heater according to Claim 1 or 2, in which series connection between the first and second portions is effected by way of a connector mounted on the dish.

5. A heater according to any preceding Claim, in which the first and second portions comprise the same metal or metal alloy composition.

6. A heater according to Claim 5, in which the metal alloy composition comprises nickel-chromium or iron-chromium-aluminium.

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7. A heater according to any preceding Claim, in which the elongate strip or ribbon of the first portion is corrugated in form.

8. A radiant electric heater constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.