GB2309874A

GB2309874A - Electrical heating elements

Info

Publication number: GB2309874A
Application number: GB9602104A
Authority: GB
Inventors: James S Birnie; Richard J Francis
Original assignee: Emerson Electric Co
Current assignee: Emerson Electric Co
Priority date: 1996-02-02
Filing date: 1996-02-02
Publication date: 1997-08-06
Also published as: WO1997028670A1; GB9602104D0

Abstract

To manufacture an electrical heating element, a heating member 10 is located in the bore of a tube 12, insulating material 14 electrically insulating the heating member from the tube, and the external surface of the tube is provided with integral projections 14 whereby to extend the surface area of the external surface of the tube. The integral projections 14 may be either helical or circular form and the insulating material is preferably compacted magnesium oxide in powdered form. The preferred method of compaction is by reducing the diameter of the tube while rolling projections in the form of fins into the surface of the tube. A composite material is used for the terminal pin 11 or dead end of the electrical heating element. The composite material consists of a good electrical conductor, such as a noble metal, eg gold or silver, or copper, encased within a sheath of another, stronger material such as mild steel or stainless steel or other suitable alloy.

Description

Electrical Heatina Element The present invention relates to electrical heating elements and more particularly to a method of constructing such elements and the materials used.

Electrical heating elements are in many forms but one common form is an immersion type element where electrical resistance heating is used to heat an intermediate member in intimate contact with the substance to be heated. The present invention will be described in relation to such an element. It should be understood, however, that aspects of the present invention are of general application to electrical heating elements.

It is known that the efficiency of heat transfer from an electrical heating element to its surroundings is a function of, inter alia the surface area of the extension of the element. Also, where the heating element is constructed from a resistance wire mounted in a thermally conductive tube filled in electrically resistive but heat conductive material, the packing density or homogeneity of the filling material is also important. Until now, these two factors have been seen as separate issues which should be separately addressed but such an approach tends to increase costs.

The present invention provides a method of manufacturing an electrical heating element comprising locating a heating member in the bore of a tube, electrically insulating the heating member from the bore of the tube, and providing the external surface of the tube with integral projections whereby to extend the surface area of the external surface of the tube.

The integral projection may be either helical or circular form and the insulating material in preferably powdered form. The preferred method of compaction is by reducing the diameter of the tube while rolling the fins into the surface of the tube.

A further aspect of the present invention is the use of a composite material for the terminal pin or dead end of the electric heating element. The composite material consists of a cored wire with the wire made from a good electrical conductor such as a nobel metal, e.g. gold or silver, or copper encased within a tube of another material such as mild steel or stainless steel or other suitable alloy.

In order that the present invention be more readily understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 shows diagrammatically a side view of a part of a heating element according to the present invention; and Fig. 2 shows a side view of a terminal pin used in the element shown in Fig. 1.

As shown in Fig. 1, an electric heating element comprises a resistance wire 10 which is electrically connected to a terminal pin 11 in any suitable manner e.g. by resistance welding and centred in a metal tube 12 provided with integral external projections 14 which provide an extended surface area for the tube 12. The projections are of any suitable form but circular or helical projections are preferred as they are most easily produced. The tube 12 is filled with an electrically insulative but thermally conductive material 15 such as magnesium oxide which is preferably in powdered form.

The terminal pin 11 is of composite construction as is shown more clearly in Fig. 2. The pin 11 comprises an outer sheath 17 of a suitable material for imparting rigidity to the pin and for ease of connection to surrounding materials and an inner core 18 of a good electrical conductor such as copper or a nobel metal such as gold or silver. The outer sheath can be mild steel or stainless steel or other alloy.

The use of a composite material for the terminal pin permits much greater currents to be used for the heating element without there being an increase in the temperature of the pin itself.

In manufacture, the terminal pin is formed by inserting a copper wire into a sleeve of mild steel and then swaging down the composite rod to a suitable diameter which can then be thread rolled and/or machined to form a terminal pin as shown in Fig. 2.

The ratio of copper core to outer sheath is not fixed but is dependent upon a number of factors including finished mechanical strength. This is because during manufacture of an electric heating element, the pip at the end of the pin is used to suspend the resistance wire assembly in the element filling machine.

Once the element is filled, the element is swaged down in order to compact the filling material and ensure rapid heat transfer and to enhance the characteristics of the electrical insulation. The outer surface of the tube of the heating element is also machined in order to form the integral projections.

This can be done either at the same time as the compacting step or as a separate subsequent step. In any event, it is to be expected that the length of the element will increase by approximately 10% during these operations.

The above methods produces an electrical heating element of predetermined characteristics in a convenient manner. Due to the fact that the surface area of the exterior of the tube is extended in such a manner that any projections from the main, root diameter of the tube are integral with the tube, the heating element is robust and thermally efficient in use.

Claims

CLAIMS:

1. A method of manufacturing an electrical heating element comprising locating a heating member in the bore of a tube, electrically insulating the heating member from the bore of the tube, and providing the external surface of the tube with integral projections whereby to extend the surface area of the external surface of the tube.

2. A method according to claim 1, wherein the integral projections are provided by rolling a helical fin into the surface of the tube.

3. A method according to claim 1, wherein the integral projections are provided by rolling circular fins into the surface of the tube.

4. A method according to claim 1, 2 or 3 wherein the heating member is electrically insulated from the tube by filling the bore of the tube with powdered material.

5. A method according to any one of the preceding claims, and comprising compacting the insulation material by reducing the diameter of the tube.

6. A method of manufacturing an electrical heating element substantially as hereinbefore described.

7. An electrical heating element when manufactured according to any one of the preceding claims.

8. An electrical heating element comprising a heating member located in the bore of a tube and electrically insulated therefrom by insulating material, the external surface having integral projections whereby to extend the surface area of the external surface of the tube.

9. A heating element according to claim 8, wherein the integral projections are in the form of a helical fin.

10. A heating element substantially as hereinbefore described with reference to the accompanying drawings.

11. The use of a composite electrically conductive member as a terminal pin for an electrical heating element.

12. The use of a composite member according to claim 11, wherein the member consists of an inner core of a good electrically conductive material and an outer sheath of a less good electrically conductive material but mechanically stronger material.

13. The use of a composite member according to claim 12, wherein the core is of copper and the outer sheath is of mild or stainless steel.