Electric Heating Device

Abstract

An elongate heating device comprises a preformed elongate core having at least one helical groove in its outer surface and a separately formed sleeve of an internal diameter greater than the external diameter of said surface of the core which surrounds the core and has in its inner surface at least one helical groove that is of the same pitch as that of the groove on the core and that defines with the groove on the core a helical passage, the core and/or the sleeve being of a metal of high thermal conductivity. A length of mineral insulated heating cable extends along the helical passage between the core and the sleeve with at least one of its ends protruding from the device. The cable is so clamped between the core and the sleeve that there is an effective heat transfer path between the cable sheath and the core and/or between the cable sheath and the sleeve.

Description

This invention relates to electric heating devices and is particularly concerned with electric heating devices of the kind incorporating, as a heating element, a mineral insulated heating cable comprising one or more bare conductors enclosed in a metallic sheath of high thermal conductivity and electrically insulated from the sheath and, where there is more than one conductor from one another, by compacted powdered mineral insulating material, usually but not necessarily magnesium oxide.

It has been proposed to manufacture an electric heating device of this kind of substantially uniform cross-section, for instance a tubular heating roll for use in the textile industry, by winding a length of mineral insulated heating cable on a tubular core, positioning the core carrying the heating cable in an annular mould with an end or ends of the cable projecting from the mould, and casting a metal of high thermal conductivity about the core to form a cast heating roll having an embedded mineral insulated heating element. We have found that when manufacturing heating rolls by this method it is difficult to avoid a relatively large proportion of rejects due to inadvertent damage to the heating element during the casting process or to other causes. Moreover heating rolls made by this method have the serious disadvantage that in the event of a fault subsequently developing in the heating element the heating roll itself has to be scrapped.

It is the object of the present invention to provide an improved method of manufacturing an elongate heating device incorporating a mineral insulated heating element in which the risk of rejects during manufacture is substantially reduced. The invention also includes an elongate heating device made by the aforesaid method which has the important advantage, that in the event of a fault developing in the heating element, the heating element can be quickly and easily replaced.

According to the invention the method comprises forming at least one helical groove in an outer surface of an elongate core; forming at least one helical groove of the same pitch as that of the groove on the core in the inner surface of a sleeve having an internal diameter which is greater than the external diameter of said surface of the core, the core and/or the sleeve being of a metal of high thermal conductivity; winding a length of mineral insulated heating cable in the groove in the core or in the groove in the sleeve to form a male thread; and screwing the core and the sleeve together so that one or each end of the cable protrudes from the heating device so formed and the cable is so clamped between the core and the sleeve that an effective heat transfer path is provided between the cable sheath and the core and/or between the cable sheath and the sleeve.

The core or the sleeve may be of a material of low thermal conductivity, for example a ceramic material. It will be appreciated that by appropriate selection of the materials of the core and of the sleeve a major proportion of the heat emitted by the heating device of the present invention may be encouraged to flow radially outwardly only, radially inwardly only or both radially outwardly and radially inwardly.

Preferably where the heating cable is of circular cross-section the or each groove in the core and the or each groove in the sleeve are each of a cross-section approximating to that of a semi-circle having a radius equal to or slightly greater than half the overall diameter of the heating cable.

The mineral insulated heating cable will normally, but not necessarily, be cut to a predetermined length before it is wound on the core. The cable employed preferably has two conductors embedded in the compacted insulation and in this case exposed ends of the conductors at one end of the length of cable are electrically connected together and enclosed in a sealed termination generally known as the "hot end" termination, and this sealed end of the cable is welded, brazed or otherwise secured in the groove in the core or in the groove in the sleeve at or near one end of the groove. Preferably the cable is welded, brazed or otherwise secured in the groove in the core or in the groove in the sleeve at or near the other end of the groove. The other end of the cable may be arranged to protrude through an end wall of the core where the cable is sealed, preferably by means of a conventional sealing pot, this sealed end of the cable generally being termed the "cold end" termination.

In an alternative construction the outer surface of the core and the inner surface of the sleeve may each have at least two helical grooves of substantially the same pitch formed in it, each of the grooves of the core being of the same pitch as the complementary groove in the sleeve. Where the core and sleeve each has two grooves a single length of mineral insulated cable may be folded back between its ends to form a doubled length of cable which is wound in the two grooves in the core or in the sleeve with the folded end of the cable welded or otherwise secured in a recess in the core or sleeve at or near one end of the grooves and with both ends of the cable protruding from the same end of the device. Where the core and sleeve each has two or more grooves, two or more separate single lengths of mineral insulated cable each having two conductors may be wound in the grooves in the core or sleeve, the conductors of one cable being of a different cross-sectional area from that of the conductors of the other cable or cables thereby providing two or more heating elements that may be used separately or in combination.

The core may be of solid form but where the heating device is intended to be mounted on or to constitute an injection nozzle or where the heating device is intended for use as a heater for a textile roller the core is, or preferably is, of tubular cross-section. The cable sheath is preferably made of stainless steel and the core and/or sleeve are each preferably made of mild steel.

The invention also includes an elongate heating device comprising a preformed elongate core having at least one helical groove in its outer surface; a separately formed sleeve of an internal diameter greater than the external diameter of said surface of the core which surrounds the core and has in its inner surface at least one helical groove that is of the same pitch as that of the groove on the core and that defines with the groove on the core at least one helical passage, the core and/or sleeve being of a metal of high thermal conductivity; and a length of mineral insulated heating cable which extends along the helical passage between the core and the sleeve with one or each of its ends protruding from the device and which is so clamped between the core and the sleeve that there is an effective heat transfer path between the cable sheath and the core and/or between the cable sheath and the sleeve.

The invention will be further illustrated by a description, by way of example, of two forms of electric heating device with reference to the accompanying drawings, in which:

FIG. 1 is a sectional side view of an electric roll heater unit for use in the textile industry;

FIG. 2 is an end view of the roll heater shown in FIG. 1;

FIG. 3 is a sectional side view of an electrically heated injection nozzle for a die casting machine;

FIG. 4 is an end view of the heated injection nozzle shown in FIG. 3,

FIG. 5 is a sectional side view of a second form of electrid roll heater unit for use in the textile industry, and FIG. 6 is an end view of the roll heater shown in FIG. 5.

Referring to FIGS. 1 and 2 the roll heater unit comprises a tubular core 1 of mild steel of circular cross-section having at one end a radially outwardly extending flange 2 in which are a plurality of holes 3 for use in mounting the unit on a support. The core 1 has a bore of circular cross-section. Formed in the outer cylindrical surface of the core 1 is a helical groove 4 of semi-circular cross-section having a radius equal to or slightly greater than half the overall diameter of the mineral insulated heating cable that is to be wound in the groove. At the end of the groove adjacent to the flange 2 a part 5 of the groove extends in a lengthwise direction to a hole 6 in the flange.

A predetermined length of mineral insulated heating cable 11 having two conductors and a stainless steel sheath is sealed at one end to form a "hot end" termination 12 and is sealed at the other end by a conventional sealing pot 13 incorporating a ceramic and metal or glazed high temperature seal to form a "cold end" termination. the "hot end" 12 of the cable 11 is slipped through the hole 6 and the cable is pulled through the hole until the sealing pot 13 abuts the end face of the flange 2 and an adjoining length of the cable lies in the part 5 of the groove 4. The cable is welded or brazed in the groove 4 at 14. The cable is tightly wound in the helical groove 4 and the "hot end" 12 of the cable is welded or brazed in the groove at 15 to form a male thread on the core.

A sleeve 8 of mild steel whose bore is stepped near one end has a helical groove 9 formed in the internal surface of a major part of the bore that is of internal diameter greater than the external diameter of the core 1, the groove having the same pitch and cross-section as the groove 5. The sleeve 8 is screwed on the threaded core 1 with an end of the sleeve abutting the flange 2 and a radially inwardly extending flange 10 on the other end of the sleeve abutting the other end of the core.

Due to the very small clearance between the mineral insulated cable 11 and the surrounding metal of the core 1 and of the sleeve 8, which clearance is substantially reduced as expansion of the cable sheath takes place when the roll is heated, an effective heat transfer path is provided between the cable sheath and the core and between the cable sheath and the sleeve.

The heated injection nozzle shown in FIGS. 3 and 4 comprises a tubular core 21 of mold steel having over an intermediate major part of its length a cylindrical surface of circular cross-section and at one end an outlet 22. The core 21 has a bore of circular cross-section whose diameter increases smoothly at the outlet 22. A helical groove 24 is formed in the outer cylindrical surface of the intermediate major part of the core 21, the groove being of a semi-circular cross-section having a radius equal to or slightly greater than half the overall diameter of the mineral insulated heating cable that is to be accommodated in the groove.

A sleeve 28 of mild steel whose bore is stepped near one end has a helical groove 29, of the same pitch and cross-section as the groove 24 on the core 21, formed in the internal surface of a major part of the bore that is of internal diameter slightly greater than the external diameter of the core. The sleeve 28 has a radially inwardly extending flange 30 at one end. A predetermined length of mineral insulated heating cable 31 having two conductors and a stainless steel sheath is sealed at one end to form a "hot end" termination 32 and is sealed at the other end by a conventional sealing pot 33 incorporating a ceramic and metal or glazed high temperature seal to form a "cold end" termination. The "hot end" 32 of the cable 31 is welded or brazed in the helical groove 29 at 35 and the cable is tightly wound in the groove to form a male thread in the bore of the sleeve, the cable also being welded or brazed in the groove at 34. The core 21 is screwed into the threaded sleeve 28 with the step between the intermediate major part of the core and the outlet 22 abutting the flange 30 and with a short length of the cable 31 with the sealing pot 33 attached to its end protruding from the other end of the nozzle.

As in the electric roll heater unit the very small clearance between the mineral insulated cable 31 and the surrounding metal of the core 21 and of the sleeve 28, substantially reduced as the nozzle is heated and expansion of the cable sheath takes place, provides an effective heat transfer path between the cable sheath and the core and between the cable sheath and the sleeve.

The roll heater unit shown in FIGS. 5 and 6 comprises a tubular core 41 of mild steel of circular cross-section having at one end a radially outwardly extending flange 42 in which are a plurality of holes 43 for use in mounting the unit on a support. the core 41 has a bore of circular cross-section. Formed in the outer cylindrical surface of the core 41 are two helical grooves 44, 44' of substantially the same pitch each of semi-circular cross-section having a radius equal to or slightly greater than half the overall diameter of the mineral insulated heating cables that are to to be wound in the grooves. At the ends of the grooves adjacent to the flange 42 are parts 45, 45' of the grooves, each of which extends in a lengthwise direction to holes 46, 46' in the flange.

Two predetermined lengths, of mineral insulated heating cable 51, 51', each having two conductors and a stainless steel sheath, are sealed at one end to form "hot end" terminations 52, 52' and are sealed at the other end by conventional sealing pots 53, 53' incorporating ceramic and metal or glazed high temperature seals to form "cold end" terminations. The cable 51 is tightly wound in the groove 44 and is welded or brazed in the groove at 54 and 55. The cable 51' is tightly wound in the groove 44' and is welded or brazed at 54' and 55'.

A sleeve 48 of mild steel whose bore is stepped near one end, has two helical grooves 49, 49' in the internal surface of a major part of the bore, each groove being of substantially the same pitch and cross-section as the grooves 44, 44' in the core 41. The sleeve 48 is screwed on the thread core 41 with an end of the sleeve abutting the flange 42 and a radially inwardly extending flange 50 on the other end of the sleeve abutting the other end of the core.

The conductors of the cable 51' are of a different cross-sectional area from that of the conductors of the cable 51, thereby providing two heating elements that may be used separately or in combination.

Electrical heating devices in accordance with the present invention can be manufactured from any suitable metal of high thermal conductivity in which it is possible to machine helical grooves and the upper temperature limit of the device is limited only by the metals used. For example, for a stainless steel sheathed cable enclosed between a core and sleeve of mild steel a maximum temperature of approximately 750.degree.C is possible. In addition to the advantage that the heating cable can be quickly and easily replaced, electric heating devices of the present invention can be assembled by unskilled or semi-skilled labour with virtually no rejections due to assembly error.

Claims

What I claim as my invention is:

1. An elongate heating device comprising a preformed elongate core having at least one helical groove in its outer surface; a separately formed sleeve of an internal diameter greater than the external diameter of said surface of the core which surrounds the core and has in its inner surface at least one helical groove that is of the same pitch as that of the groove on the core and that defines with the groove on the core a helical passage, at least one of the core and the sleeve being of a metal of high thermal conductivity; a length of mineral insulated heating cable comprising an elongate metallic sheath of high thermal conductivity, at least one elongate bare electrical heating conductor in said metallic sheath and compacted powdered mineral insulating material sealing the space between the electric heating conductor and the metallic sheath and insulating said conductor from said metallic sheath, which cable extends along the helical passage between the core and the sleeve with at least one of its ends protruding from the device, and electrical terminal means connected to each protruding end of the cable, the cable being so clamped between the core and sleeve and being of such a diameter with respect to the depth of the grooves, that part of the cable fits in the groove in the core and part of the cable fits in the groove in sleeve and that there is an effective heat transfer path between at least one of the cable sheath and the core, and the cable sheath and the sleeve.

2. An elongate heating device as claimed in claim 1, wherein the outer surface of the core and the inner surface of the sleeve each have at least two helical grooves of substantially the same pitch formed in it, each of the grooves of the core being of the same pitch as the complementary groove in the sleeve and wherein at least two separate single lengths of mineral insulated heating cable each having two conductors extend in the helical passages defined by the grooves, the conductors of one cable being of a different cross-sectional area from that of the conductors of the other cable or cables thereby providing at least two heating elements that may be used separately or in combination.

3. An elongate heating device as claimed in claim 1, wherein the mineral insulated cable is of circular cross-section and each groove is of a semi-circular cross-section having a radius substantially equal to half the overall diameter of the cable.

4. An elongate heating device as claimed in claim 1, wherein one of the core and the sleeve is of a material of low thermal conductivity.

5. An elongate heating device as claimed in claim 1, wherein the terminal means connected to each protruding end of the heating cable is a sealing pot.

6. An elongate heating device as claimed in claim 1, wherein the core is of tubular cross-section.

7. An elongate heating device as claimed in claim 1, wherein the sheath of the heating cable is of stainless steel and at least one of the core and the sleeve is of mild steel.

8. An elongate heating device comprising a preformed elongate core having at least one helical groove in its outer surface; a separately formed sleeve of an internal diameter greater than the external diameter of said surface of the core which surrounds the core and has in its inner surface at least one helical groove that is of the same pitch as that of the groove on the core and that defines with the groove on the core a helical passage, at least one of the core and the sleeve being of a metal of high thermal conductivity; a length of mineral insulated heating cable comprising an elongate metallic sheath of high thermal conductivity, two elongate bare electrical heating conductors in said metallic sheath and compacted powdered mineral insulating material sealing the space between the electric heating conductors and the metallic sheath and insulating said conductors from each other and from said metallic sheath, said heating conductors being electrically connected together at one end of the lengths and enclosed in a sealed termination at said end, which cable extends along the helical passage between the core and the sleeve with said sealed termination secured in the passage near one end of the passage and with the other end of the cable protruding from the device and electrical terminal means connected to the protruding end of the cable, the cable being clamped between the core and sleeve and being of such a diameter with respect to the depths of the grooves, that part of the cable fits in the groove in the core and part of the cable fits in the groove in the sleeve and that there is an effective heat transfer path between at least one of the cable sheath and the core, and the cable sheath and the sleeve.

9. An elongate heating device as claimed in claim 8, wherein the sealed termination at one end of the length of mineral insulated heating cable is welded in the groove in the core or sleeve.