GB2201361A

GB2201361A - Manufacturing a heating element

Info

Publication number: GB2201361A
Application number: GB08629922A
Authority: GB
Inventors: Allen Albert Brzeczek
Original assignee: Heatrod Elements Ltd
Current assignee: Heatrod Elements Ltd
Priority date: 1986-10-08
Filing date: 1986-12-15
Publication date: 1988-09-01
Anticipated expiration: 2006-12-15
Also published as: GB8624131D0; GB8629922D0; GB2201361B

Abstract

The method of manufacturing a heating element of the type having a spiral heating element 12 contained within an outer conductive sheath 10 which is packed around the spiral heating element with powdered insulating material 18 such as magnesia has the ends of the sheath tube closed by end plugs 20, 22 prior to the conventional rolling and annealing steps. This is made possible by the use of dry magnesia, conveniently silicon treated magnesia, which does not require subsequent heating to drive off moisture, and by forming the end plugs of a resiliently deformable material, such as high temperature silicon rubber, which is capable of withstanding the temperature experienced during the subsequent annealing step. <IMAGE>

Description

Title: Improvements in and relating to heating elements.

Description.

Field of invention.

This invention reiates to the design and construction of heating elements particularly although not exclusively of the type employed in full flow water heaters for shower units.

Background to the invention.

Hitherto the construction of such heating elements has involved the following steps: 1. A spiral element has been threaded into a small bore tube of metal such as copper which latter is to form the outer sheath of the heating element, the spiral not at any point touching the tube wall.

2. The tube has been filled with magnesium oxide powder to fill the space in the tube not occupied by the spiral.

3. The tube diameter has been reduced by rolling to compress the powder and to increase its thermal conductivity.

4. The assembly has been heated to a high temperature typically in excess of 3650C to anneal the sheath material following work hardening by the rolling process.

5. The annealed tube has been shaped as by bending to form the desired eventual shape of the element, typically a spiral and fixings (if any) such as securing brackets, lugs and the like have been brazed or soldered to the element.

6. The assembly has then been re-heated to an elevated temperature typically in the range of 200-300 C to dry out the magnesium oxide powder. Typically this has required heating for 4 to 6 hours or longer.

7. After cooling to typically 1200C, silicon resin has been applied as quickly as possible to the exposed ends of the powder in the tube to seal the powder against the ingress of moisture and residual heat in the tube has in part served to cure the resin. Further heat has from time to time been necessary to fully cure the resin.

8. Liquid silicon rubber has then been injected into the ends of the tube and a ceramic bead fitted as an insulating plug at each end, the silicon rubber serving as an adhesive and a further barrier to moisture ingress.

The above process has the disadvantage that sealing against moisture ingress of water is not possible until after the heating process for annealing the sheath material has been completed due to degradation of the sealing materials employed.

It is an object of the present invention to provide an improved method for the manufacture and assembly of such heating elements which avoids this problem.

Summary of the invention.

According to one aspect of.the present invention the method of manufacture as aforesaid is modified by the use of a powder which is sufficiently dry when inserted not to need subsequent heating to drive off surplus moisture, and the use of a material for the plugs for sealing the ends of the powder column which is capable of withstanding elevated temperatures consistent with annealing the material from which the outer tube is formed to enable annealing to be performed after the powder has been sealed in place in the tube.

Advantageously the plug material is of sufficient mechanical strength and has sufficiently good electrically insulating characteristics as to obviate the need for the ceramic beads of the prior art method.

The invention thus lies in the following method of manufacture, namely: 1. A metal tube is first cut to length and a restriction in the internal tube diameter such as is obtained by a discontinuous rill, is formed near to one end thereof.

2. A sleeve of resiliently deformable material capable of withstanding high temperatures and having good electrical and water penetration resistance is fitted to one terminal end of a small diamet:er heating spiral and the other end of the spiral is threaded into and through the tube until the sleeved end is pulled into engagement with the restriction in the tube.

3. The heating spiral is then tensioned using the engagement of the sleeved end thereof with the restriction in the tube as an end-stop, and with the tube in a generally upright condition with the restriction lowermost, the tube is filled with a dry electrically insulating but thermally conductive powder (typically magnesium- oxide and preferably silicon treated magnesium oxide).

4. A second sleeve of similar material to the first mentioned sleeve is then fitted over the protruding upper terminal end of the heating spiral and is pushed into the upper end of the tube so as to fill the space around the protruding terminal end.

5. The tube is then rolled to reduce its diameter to obtain optimum powder compaction (ie to obtain a packing density appropriate to the thermal conductivity required but not too great so that fissures will occur in the powder when the filled tube is rolled, annealed andbent.) 6. The act of rolling the tube squeezes the silicon rubber sleeves so that each sleeve protrudes axially from the ends of the tube to form an electrically insulating bead around the protruding terminal ends, thereby obviating the need for ceramic beads to be cemented to the terminal ends as in the prior art method.

7. The sealed tube is then annealed throughout its complete length by heating at an elevated temperature for a sufficient period of time, the temperature being selected so as to be high enough to anneal the metal but not too high to degrade the silicon rubber.

8. Any fixing brackets or the like are then secured to the tube by brazing or silver soldering or the like, care being taken to ensure that the temperature of any such process is again less than that at which degradation of the silicon rubber will occur.

A preferred powder material is silicon treated magnesium oxide, typically magnesium oxide which has been treated with methylhydrogenpolysiloxane fluid to seal the hygroscopic magnesium oxide particles.

A preferred material for the end plugs is high temperature silicon rubber as sold by Silicone-Altimex Ltd. of Nottinqham under the type number SA155. This material has been found to be capable of withstanding temperatures up to 3650C for sustained periods of time without apparent degradation occurring.

Where the tube material is copper the annealing process is typically carried out at 3650C for up to nine hours.

Where the tube material is aluminium the annealing process is typically carried out at 3600 for a period of time of the order of 15-30 minutes, typically 20 minutes.

The invention also lies in a heating element when made in accordance with the above method and in a water heater when fitted with a heating element constructed as aforesaid. The aforedescribed heating element may aiso be employed, however, in low temperature air heating applications, whether cast in or in free air.

According to another aspect of the present invention a heating element typically although not exclusively for use in a water heater or the like such as is employed for heating water as it is supplied to a shower head, comprises: 1. an outer metal sheath containing a spiral heating element and packed with a dry electrically insulating but thermally conductive powder with terminal ends of the heating element protruding through and beyond the ends of the sheath, and 2. end plugs of temperature resistant electrically insulating material held captive in the ends of the sheath and serving to (a) space and insulate the conductive terminal ends of the spiral heating element from the sheath, and (b) seal the powder against the ingress of moisture.

Preferably the powder is silicon treated magnesium oxide.

Preferably the temperature resistant material from which the plugs are formed is silicon rubber as supplied under the -Grade SA155 by Silicone-Altimex Ltd..

The invention will now be described by way of example with reference to the accompanying drawings in which: Fig. I is a side view of an elongate heating element constructed in accordance with the invention, Fig. 2 is a cross section through the element of Fig. 1, Fig. 3 is a side view of the element after bending to form a coil, and Fig. 4 is an end view of the coil shown in Fig. 3.

Detailed description of the drawings.

In Figs. 1 and 2 a tube of copper 10 is shown fitted centrally with a heating spiral 12 which itself is secured at opposite ends to a pair of conductive terminals 14,16 which protrude through the opposite ends of the tube 10.

The interior of the tube is packed with silicon treated magnesium oxide powder 18 and the ends of the tube are closed by two sleeves of silicon rubber 20,22 which as shown are held captive in the ends of the tube by a reduction in the diameter of the tube brought about by rolling the tube after the two sleeves have been inserted following the filling of the tube with powder.

The final form of the heating element is shown in Figs. 3 and 4 where the elongate tube 10 has been bent around in the form of a coil designated 24 in Fig. 3. with the two ends of the coil 26 and 28 bent through a right-angle to protrude in an axially parallel direction at one end thereof.

Claims

A A method of manfacturing a heating element comprising the following steps: 1. A spiral element is threaded into a metal tube which is to form the outer sheath of the heating element, with the spiral not in contact with the tube; 2. The tube is filled with powdered insulating material such as magnesium oxide sufficiently dry not to require subsequent heating to drive off moisture; 3. The ends of the tube are closed with resiliently deformable end plugs; 4. The tube is rolled to reduce its diameter and is then annealed, the end plugs being-constituted by a material which is capable of withstanding the elevated temperature required for the annealing step.
2 A method of manufacturing a heating element which comprises the following steps: 1. A metal tube cut to length is formed with an internal restriction adjacent one end thereof; 2. A resiiiently deformable sleeve capable of withstanding high temperatures is fitted to one terminal end of a heating spiral and the ot:her end of the spiral. is pulled into and through the tube until the sleeved end engages the restriction in the tube;
3. The heating spiral is tensioned and the tube filled with a dry powdered insulating powder such as magnesium oxide; 4. A second resiliently deformable sleeve capable of withstanding high temperatures is fitted to the protruding terminal end of the heating spiral and inserted into the other end of the tubé so as to fill the space around the protruding terminal end; 5.The tube is rolled to reduce its diameter so that the resiliently deformable sleeves are caused to protrude axially from the ends of the tube to form electrically insulating beads around the respective protruding terminal ends; 6. The sealed tube is annealed at annealing temperature which does not adversely affect the resiliently deformable sleeves sealing the ends of the sheath.

/3 A method according to claim 1 or claim 2 in which the powdered insulating material used to fill the tube is silicon treated magnesium oxide.
4 A method according to claim 3, in which t:he powdered magnesium oxide is treated with methylhydrogenpolysiloxane.

/
5 A method according to any one of the preceding claims, in which the resiliently deformable end plugs or sleeves are made of high temperature silicon rubber, preferably capable of withstanding temperatures of at least 3650C.
> A method according to any of the preceding claims wherein the sheath tube is made of copper and the annealing step is carried out at about 3650C for up to nine hours.
7 A method accordng to any one of claims 1 to 5, in which the sheath tube is made of aluminium and the anneaiing step is carried out at about 3600C for a period in the range 15 to 30 minutes.
8. A heating element which comprises; 1. An outermetal sheath containing a spiral heating element and packed with a dry electrically insulating but thermally conductive powder with terminal ends of the heating element protruding through and on the ends of the sheath, and 2. end plugs of temperature resistant electrically insulating material held captive in the ends of the sheath and serving firstly to space and insulate the conductive terminal ends of the spiral heating element from the sheath and secondly to seal the powder against ingress of moisture.
9 A heating element as claimed in claim 8 in which the insulating powder with which the sheath is packed is constituted by silicon treated magnesium oxide.
1 > A heating element as claimed in claim 8 or claim 9 in which the end plugs are formed of silicon rubber, preferablysilicon rubber capable of withstanding temperatures up to 3650C for sustained periods of time.
1@. A method of manufacturing a heating element substantially as herein before described.
12. A heating element substantially as herein before descried with reference to the accompanying drawings.

12@ A heating element substantially as herein before described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS 1. A method of ma-nfacturing a heating element comprising the followi ps: (a). a spiral element is threaded into a metal tube which is to form lile ojher sheath of the heating element, with the spiral not in contact with the tube; (b). the tube is filled with powdered insulating material such as magnesium oxide sufficiently dry not to require subsequent heating to drive oEf moisture; (c). the ends of the tube are closed with resiliently deformable end plugs;; (d). the tube is rolled to reduce its diameter and is then annealed, the end plugs being constituted by a material which is capable of withstagiing the elevated temperature required for the annealing step.

2. A method of manufacturing a heating element which comprises the following steps: (a). a metal tube cut to length is formed with an internal restriction adjacent one end thereof; (b). a resiliently deformable sleeve capable of withstanding high temperatures is fitted to one terminal end of a heating spiral and the other end of the spiral is pulled into and through the tube until the sleeved end engages the restriction in the tube; (c). the heating spiral is tensioned and the tube filled with a dry powdered insulating powder such as magnesium oxide; (d). a second resiliently deformable sleeve capable of withstanding high temperatures is fitted to the protruding terminal end of the heating spiral and inserted into the other end of the tube so as to fill the space around the protruding terminal end;; (e). the tube is rolled to reduce its diameter so that the resiliently deformable sleeves are caused to protrude axially from the ends of the tube to form electrically insulating beads around the respective protruding terminal ends; and (f). the sealed tube is annealed at annealing temperature which does not adversely affect the resiliently deformable sleeves sealing the ends of the sheath.

3. A method according to clairn 1 or claim 2 in which the powdered insulating material used to fill the tube is silicon treated magnesium oxide.

4. A method according to claim 3, in which the powdered magnesium oxide is treated with methylhydrogenpolys iloxane.

5. A method according to any one of the preceding claims, in which the resiliently deformable end plugs or sleeves are made of high temperature silicon rubber, preferably capable of withstanding temperatures of at least 365"C.

6. A method according to any of the preceding claims wherein the sheath tube is made of copper and the annealing step is carried out at about 365"C for up to nine hours.

7. A method according to any one of claims 1 to 5, in which the sheath tube is made of aluminium and the annealing step is carried out at about 3600C for a period in the range 15 to 30 minutes.

8. A heating element which comprises; (a). an outer metal sheath containing a spiral heating element and packed with a dry electrically insulating but thermally conductive powder with terminal ends of the heating element protruding through and on the ends of the sheath, and (b). end plugs of electrically insulating material held captive in the ends of the sheath and serving firstly to space and sS e i: :he conductive terminal ends of the spiral heating element from the sheath and secondly to seal the powder against ingress of moisture, wherein (c). the Xnetal~sheath has been produced by rolling a metal tube to reduce its diameter and then annealing with the insulating packing powder and end plugs in place, the end plugs being of an insulating material sufficiently temperature resistant to withstand the elevated temperature required or annealing.

9. A heating element as claimed in claim 8 in which the insulating powder with which the sheath is packed is constituted by silicon treated magnesium oxide.

10. A heating element as claimed in claim 8 or claim 9 in which the end plugs are formed of silicon rubber, preferably silicon rubber capable of withstanding temperatures up to 365 C for sustained periods of time.

11. A method of manufacturing a heating element substantially as herein before described.