GB2048626A - An electrical heating tape - Google Patents

Abstract

A woven electrical heating tape (1) comprises an electrical heating element (2) as the weft. The element may be a single strand of a fine resistance alloy or a wire wound as a micro-helix on a small diameter core. Busbars (3) extend along the tape edges. These busbars (3) are stranded and connected to the heating element at regular longitudinal intervals. The busbars may be helical, in which case the element is held between adjacent turns of the helix at connection points, or braided, in which case the element is woven into the busbars at connection points. Additional busbars can be provided as required. <IMAGE>

Description

SPECIFICATION An electrical heating tape The present invention relates to an electrical heat ing tape used to heat the surface of a body in order to raise or maintain the temperature of that surface or body.

Electric heating tape units designed to produce a particular heat output at a designated voltage and temperature, with or without cold leads are already known.

These devices are traditionally produced using resistance wires or foils enclosed within a fabric, plastic or rubber covering with the resistance wires arranged electrically to form a complete flexible heating unit with predetermined characteristics of length and output wattage.

It is known that a greater convenience for certain applications may result from an ability to cut a particular length of heating tape from a roll of considerable total length and use the portion cut-off as an independent unit with known performance characteristics.

This may be achieved by replacing the metallic resistance elements of a traditional unit by a specially formulated plastic material or by arranging for the metallic resistance elements to be in discrete longitudinal segments with each segment having predetermined electrical characteristics.

Both the above constructions require that current carrying conductors or busbars are laid parallel to the length of the heating unit connected to the resistive segments at intervals alternating from side to side.

This latter construction with parallel busbars is known as a parallel circuitry heating tape and offers cut-to-length facilities and also means that a break in the electrical path of a single segment may not result in failure of the whole trace heating unit. Certain parallel circuitry heating tape units incorporating an electrically conductive plastic compound as the resistance element exhibit self-regulating characteristics whereby self-damage due to overheating is eliminated.

The ultimate unit length of parallel circuitry electrical heating tapes, however, is limited by the capacity of the current carrying busbars which.must be de-rated to the operating temperature of the trace heating unit and not be so large as to negate the unit's flexibility.

The production of self-regulating units is complex and they can exhibit certain undesirable physical characteristics such as a tendency to remain opencircuit after exposure to high contact temperatures and draw high in-rush currents at switch-on.

According to one aspect of the present invention, there is provided an electrical heating tape comprises ing an electrical heating element passing back and forth across the width of the tape, a first pair of busbars extending longitudinally one to each side of the tape, the electrical heating element being connected to these busbars at alternate intervals therealong and additional busbars also running along the length of the tape and capable of being connected to the first mentioned busbars.

According to another aspect of the present invention, there is provided an electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape and busbars extending longitudinally of the tape, the busbars comprising a helical coil and being connected to the element at intervals along the coil, the element being received between adjacent turns of the coil at each point of connection.

According to a further aspect of the invention, there is provided an electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape and busbars extending longitudinally of the tape and connected to the element at intervals, the busbars comprising a plurality of strands with which the electrical heating element is interwoven at each point of connection.

According to a still further aspect of the invention, there is provided an electrical heating element passing back and forth across the width of the tape and busbars extending longitudinally of the tape and connected to the element at intervals, the busbars being of braid and interwoven with the heating element at each point of connection.

According to a still further aspect of the invention, there is provided an electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape and busbars extending longitudinally of the tape, the electrical heating element comprising a central textile core and a surrounding helical resistance wire or tape outer part.

According to a still further aspect of this invention, there is provided an electrical heating tape comprising heating element passing back and forth across the width of the tape connected at intervals to busbars extending longitudinally to each side of the tape, the whole disposed in an extruded electrically insulating cover to which may be attached a further extrusion of known electrical characteristics containing spaced apart electrical conductors, forming the temperature responsive component of an electronic continuous temperature monitoring circuit.

A preferred embodiment of the invention may comprise any one or more of the following advantageous features.

(a) The first busbars are disposed with the heating element in an extruded electrically insulating cover and the additional busbars are capable of being connected through this cover to the first busbars.

(b) The additional busbars are contained within an electrically insulating cover which is conveniently extruded over the electrically insulated heating unit.

(c) The additional busbars are metallic strips to increase the current carrying capacity with good thermal characteristics and give flexibility in only one plane.

(d) The first busbars are constructed of helical coils or braid to secure a mechanically sound electrical joint with the heating element at intervals and con tain thermal expansion.

(e) There is a multiplicity of coil or braid conductors comprising the first busbars which may be interwo ven with the heating element at intervals.

(f) The electrical heating element comprises the weft in a woven tape construction.

(g) The electrical heatingelementisformedfrnm a spiralled resistance element on an insulating core in the form of a single or double helix to increase the possible circuit resistance between connections and control either the wattage output or the intervals between connections or both.

(h) The temperature sensor element is contained within the first extrusion of electrical insulant over the heating tape.

(i) The temperature sensor element is placed over the insulated heating tape and contained by a second extrusion of electrical insulant which may also contain the additional busbars.

(j) A metallic foil or braid screen may be interposed between the electrically insulating heating tape and a secondarycoverofelectrical insulation for enhanced mechanical strength orto provide an electrical earth connection or both.

In order that the invention may be more clearly understood, one embodiment of the invention will now be described, by way of example, with refer encetothe accompanying drawings, in which: Figure 1 shows one form of the invention, Figure 1A is a detail of the embodiment of Figure 1, Figure 2 shows a cross sectional view of the embodiment of Figure 1, Figure 3 shows a plan view of another form of the invention, and Figure 4 shows a cross-sectional view of the arrangement of Figure 3.

Referring to Figures 1 and 2, the electrical heating tape 1 is woven and comprises as a weft an electrical heating element 2 which passes back and forth across the width of the tape. This heating element is shown here as a single strand of a fine resistance alloy whose maximum size is chosen having due regard to the necessity to bend the element back and forth as part of the normal weaving process. The element 2, however, may equally well be a microhelix of wire wound helically on to a very small diameter core to form a small diameter element having an increased resistance per unit length. This improved resistance property allows for a much shorter element length between consecutive connections with busbars 3, and a heavier resistance wire of greater mechanical strength.Achieving a short element length between consecutive busbar connections is important as this determines the maximum length of tape remaining inactive when a given length oftape is cut from a continuous reel.

The busbars 3 may take any one of a number of different forms. In Figure 1 two busbars 3 are shown extending longitudinally of the tape 1 and disposed on opposite sides respectively of the tape. Each busbar comprises three or more component strands.

The strands may be in the form of a helix or braid. At each connection point the electrical heating element is interwoven with the individual strands of the bus bar as shown in greater detail in Figure 1A. Connection points between busbars 3 and heating element 2 are disposed at regular intervals along the tape 1 alternately to one busbar and then the other. In the case of helical strands the heating element 2 is held between adjacent turns of the helices at each con nection point In the case ofthe braided construction the heating element 2 is interwoven with the braid at each connection point. The braided construction is easier to handle in the looms since the helical construction, while affording good electrical connection of the busbars to the heating element, tends to be springy and more difficult to handle.

The tape is completed by extruding over it an electrically insulating protective sheath 4 preferably of thermoplastic compound or silicone rubber.

Should an electrical heating tape of a higher cur- rent carrying capacity be required, additional busbars 5 can be provided as shown in Figures 3 and 4.

These additional busbars 5 are placed to both sides of the tape of Figures 1 and 2 and encapsulated in electrically insulating material by means of a further sheath 6. The material of this sheath will be the same as that of the sheath 4 or any other suitable material.

Also, although the additional busbars 5 have been described as alongside and to the side of the first mentioned busbars, they may be above or below the original busbars 3. The additional busbars 5 may be connected to the original busbars 3 through the insulating material of the sheaths 4 and 6. The points of connection could be the same points of connection as the element with the original busbars 3.

These points of connection would tend to stand proud of the tape. Although the additional busbars 5 have been described as being of wire form, they may be in the form of a strip or a foil. Where the additional busbars are in this form, they may also provide additional mechanical protection for the strip.

This is particularly so where they are provided on opposite faces respectively of the basic tape.

One danger with tapes of the form above described is that where a tape is broken or cut through, the break or cut may result in a live open ended conductor being produced. This can be protected against by providing an earth leakage circuit which will trip out when an earth fault occurs or, where this is not feasible, a pilot cable supplying a pilot light which goes out when the tape and with it the cable is broken or cut through. The pilot cable and light could be duplicated if desired.

In addition the tape could incorporate a known continuous temperature sensing element along its length. The sensor element in the form of two conductors at a predetermined spacing in which the impedance varies with the temperature, may be applied over the first layer of insulation 4 and then itself covered with the further sheath, 6.

It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope ofthe invention.

The above described embodiment, provides an electrical heating tape which augments the current carrying capacity of the busbars and provides a continuous surface temperature monitor. The novel application of conventional manufacturing techniques results in a product offering the main benefits of a parallel circuitry heating tape and overcoming the main known disadvantages.

Amendments to be printed under Sect. 16(1)-Prin- ters instructions Application No. 43764/78 Claims filed on 1 st November 1979

Claims (17)

1. An electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape, a first pair of busbars extending longitudinally one to each side of the tape, the electrical heating element being connected to these busbars at alternate intervals therealong and additional busbars also running along the length of the tape and capable of being connected to the first mentioned busbars.

2. An electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape and busbars extending longitudinally of the tape, the busbars comprising a helical coil and being connected to the element at intervals along the coil, the element being received between adjacent turns of the coil at each point of connection.

3. An electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape and busbars extending longitudinally of the tape and connected to the element at intervals, the busbars comprising a plurality of strands with which the electrical heating element is interwoven at each point of connection.

4. An electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape and busbars extending longitudinally of the tape and connected to the element at intervals the busbars being of braid and interwoven with the heating element at each point of connection.

5. An electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape and busbars extending longitudinally of the tape, the electrical heating element comprising a central textile core and a surrounding helical resistance wire or tape outer part.

6. An electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape connected at intervals to busbars extending longitudinally to each side of the tape, the whole disposed in an extruded electrically insulating cover to which may be attached a further extrusion of known electrical characteristics containing spaced apart electrical conductors, forming the temperature responsive component of an electronic temperature monitoring circuit.

7. An electrical heating tape as claimed in Claim 1, in which the first busbars are disposed with the heating element in an extruded electrically insulating cover and the additional busbars are capable of being connected through this cover to the first busbars.

8. An electrical heating tape as claimed in Claim 1, in which the additional busbars are contained within an electrically insulating cover which is conveniently extruded over the electrically insulated heating unit.

9. An electrical heating tape as claimed in Claim 1, in which the additional busbars are metallic strips to increase the current carrying capacity with good thermal characteristics and give flexibility in only one plane.

10. An electrical heating tape as claimed in Claim 1, in which the first busbars are constructed of helical coils or braid to secure a mechanically sound electrical joint with the heating element at intervals and contain thermal expansion.

11. An electrical heating tape as claimed in Claim 1, in which there is a multiplicity of coil or braid conductors comprising the first busbars which may be interwoven with the heating element at intervals.

12. An electrical heating tape as claimed in any preceding claim, in which the electrical heating element comprises the weft in a woven tape construction.

13. An electrical heating tape as claimed in any of Claims 1 to 4, and 6 to 12, in which the electrical heating element is formed from a spiralled resistance element on an insulating core in the form of a single or double helix to increase the possible circuit resistance between connections and control either the wattage output or the intervals between connections or both

14. An electrical heating tape as claimed in Claim 6, in which the temperature sensor element is contained within the first extrusion of electrical insulant over the heating tape.

15. An electrical heating tape as claimed in Claim 14, in which the temperature sensor element is placed over the insulated heating tape and contained by a second extrusion of electrical insulatant which may also contain the additional busbars.

16. An electrical heating tape as claimed in any preceding claim, in which a metallic foil or braid screen may be interposed between the electrically insulating heating element and a secondary cover of electrical insulation for enhanced mechanical strength orto provide an electrical earth connection or both.

17. An electrical heating tape comprising an electrical heating element passing back and forth across the width of the tape substantially as hereinbefore described with reference to Figures 1, 1A and 2 orto Figures 3 and 4 of the accompanying drawing.