GB2082429A - Resistance heating elements - Google Patents

Abstract

An electrical resistance heater comprises a resistance wire (4) wound around a magnesium former (3) and sandwiched between magnesium plates (2) held within a stainless steel sheath (1). The magnesium metal is subjected to slow oxidation under pressure between cover plates (6), resulting in expansion of the metal so as to reduce the spaces between the plates and enclose the wire (4). The former (3) could be formed from mica instead of magnesium. <IMAGE>

Description

SPECIFICATION Improvements relating to resistance heat ing elements This invention is concerned with electrical resistance heaters comprising a wire or tape of electrical resistance material wound on a for mer enclosed by protective plates. The former and plates have to be both heat resistant and electrically non-conductive and two conventional materials for this purpose are mica and ceramics. Heaters of this type using mica are limited in the amount of heat which they can withstand to a temperature usually below 350"C as mica tends to break down at 350/400"C.

It is an object of this invention to provide an electrical resistance heater which can operate at high temperatures.

Accordingly, this invention comprises a former plate of a heat resistant electrically nonconductive material on which is wound an electrical resistance element in the form of a wire or tape sandwiched between a pair of plates of magnesium and held within a containing sheath, the surfaces of the magnesium in contact with the resistance element having been converted to the oxide form into intimate contact with the resistance element.

The magnesium oxide is a non-conductive material and so meets one of the requirements for such a heater. Furthermore, it can withstand temperatures up to 750"C and so the heater can be used for high temperature applications. It is preferred that the former should also be formed from magnesium whose surfaces have been converted to the oxide form.

However mica could be used for the former as a degree of breakdown of the mica at high temperatures will not necessarily destroy the proper operation of the heater since the resistance element is enclosed by, and to an extent held by, the outer magnesium plates.

Ideally the magnesium oxide layers will have expanded to fill substantially the spaces between the windings of the resistance element, thus reducing the extent of air spaces which can act as heat insulation. The containing sheath is ideally formed from stainless or mild steel and the heater may be in the form of a flat plate or of a flat plate bent into an annulus.

The invention also extends to a method of forming an electrical resistance heater comprising winding an electrical resistance element in the form of a wire or tape onto a former plate of heat resistant and electrically non-conductive material and applying plates of magnesium metal to either side theof, enclosing the sandwich so formed in 8 containing sheath and subjecting the magnesium to slow oxidation to convert the surface layers of the magnesium to the oxide form in intimate contact with the resistance element.

Preferably the former is also of magnesium and the magnesium of each plate is ideally oxidised at high pressure in a steam atmosphere to ensure the desired degree of slow oxidation (rapid oxidation of the magnesium would lead to complete combustion of the magnesium plates). If the sandwich is gripped between cover plates under pressure during the oxidation step this will ensure that expansion of the magnesium plates, as the surfaces thereof are converted to the oxide form, will be into the spaces between the windings of the resistance element so as to fill air gaps.

The heater will ideally be dried in a drier after the oxidation step and can be shaped into an annular or other desired shape after the oxidation step.

The invention may be performed in various ways and one preferred embodiment thereof will now be described, with reference to the accompanying drawings, in which: Figure 1 is an illustrative external view of a flat plate heater constructed in accordance with this invention; Figure 2 is a cross-section through the heater of Fig. 1 during the course of formation; Figure 3 is a section through a detail of the heater of Fig. 1 after its formation; Figure 4 illustrates the conventional shape of a plate heater, but constructed in accordance with the invention; Figures 5 and 6 are vertical and horizontal sections through the plate heater of Fig. 4 showing its construction; and Figure 7 shows how the plate heater of Fig.

4 may be roiled to form a band heater.

The electrical resistance heater shown in Figs. 1 to 3 has a stainless steel containing sheath 1 which encloses a sandwich formed by outer magnesium plates 2, 2A and a central magnesium former plate 3. A nickelchrome resistor wire 4 is wound around the former sheet 3 and the ends 5 of the wire provide electrical connections for the heater.

During the formation stage of the heater the sheath 1 is gripped between a pair of cover plates 6 and the whole unit is then heated in an autoclave to a temperature of about 270/280"C at high pressure (such as 900 p.s.i.). Distilled water within the autoclave is converted to steam and causes the exposed surfaces of the magnesium plates 2 and 3 to change at a relatively slow rate to the oxide form. in the process of this change the plates 2 and 3 expand but, due to the pressure exerted by the cover plates 6, can only expand inwardly so as to fill the spaces between the windings of the wire 4 and substantially reduce the presence of air spaces. The magnesium oxide layers formed are generally illustrated by the dashed lines 7 in Fig. 3.

After removal from the autoclave the heater is dried in an oven to remove any remaining moisture and the heater can then be formed to any desired shape by rolling or compression actions applied to the sheath 1. The rolling and pressing action applied to the sheath 1 tends to compress the plates 2 and 3 even more so as to close up any remaining air gaps. For example, the heater could be formed into the shape of an annulus for use as a nozzle heater in a plastics injection moulding machine. Alternatively, the heater could be left as a plate heater.

In the heater shown in Figs. 4 to 6, an additional magnesium plate 8 is employed enabling the lead wires 5 to be brought out together at a central outlet 9 of the flat plate heater. Thus, in forming this form of heater 80/20 nichrome wire or tape 4 is wound on a pure magnesium sheet 3 of 0.5 mm thickness. One sheet 2 of pure magnesium is placed below this winding and two sheets 2A, 8 are placed above (Figure 5). The tails 5 of the winding 4 are brought between the two top sheets and connected to pure nickel leads 10. These leads emerge from the centre of the top magnesium sheet 8. This assembly is then placed in a preformed stainless steel tube 1. The assembly is then rolled to the required shape for a band heater (Fig. 7) or pressed for a plate heater (Fig. 4). When the heater is of the required shape, it is then placed in an autoclave at the appropriate temperature and pressure. This changes the surface of the pure magnesium sheet into magnesium oxide, as previously described.

The heater is then dried out and fitted with a standard (CLW Type) cap and lead output assembly 9 as shown in Figs. 4 and 7.

Claims (11)

1. An electrical resistance heater comprising a former plate of a heat resistant electrically non-conductive material on which is wound an electrical resistance element in the form of a wire or tape sandwiched between a pair of plates of magnesium and held within a containing sheath, the surfaces of the magnesium in contact with the resistance element having been converted to the oxide form into intimate contact with the resistance element.

2. A heater according to claim 1, wherein the former is constructed from mica or from a layer of magnesium whose surfaces have been converted to the oxide form into intimate contact with the resistance element.

3. A heater according to claim 1 or claim 2, wherein the magnesium oxide layers have expanded to fill substantially the spaces between the windings of the resistance element.

4. A heater according to any one of claims 1 to 3, wherein the containing sheath is formed from stainless or mild steel.

5. A heater according to any one of claims 1 to 4, in the form of a flat plate or of a flat plate bent into an annulus.

6. An electrical resistance heater substantially as herein described with reference to the accompanying drawings.

7. A method of forming an electrical resistance heater comprising winding an electrical resistance element in the form of a wire or tape onto a former plate of a heat-resistant electrically non-conductive material and applying plates of magnesium metal to either side thereof, enclosing the sandwich so formed in a containing sheath and subjecting the magnesium to slow oxidation to convert the surface layers of the magnesium to the oxide form in intimate contact with the resistance element.

8. A method according to claim 7, wherein the former also comprises a plate of magnesium on which the wire is wound prior to the oxidation treatment.

9. A method according to claim 7 or claim 8, wherein the magnesium of each plate is oxidised at high pressure in a steam atmosphere.

10. A method according to claim 9, wherein the sandwich is gripped between cover plates under pressure during the oxidation step.

11. A method according to claim 9 or claim 10, wherein the heater is dried in a drier after the oxidation step.

1 2. A method according to any one of the claims 7 to 11, wherein the heater is shaped into an annular or other desired shape either prior to or after the oxidation step.

1 3. A method of forming an electrical resistance heater substantially as herein described with reference to the accompanying drawings.