GB2138660A - Heating Devices - Google Patents

Abstract

A heating device includes a pair of parallel busbars 4 and 6 encased in respective electrically insulating sleeves 8 and 10. A resistance heating wire 12 is coiled around the electrically insulated busbars 4 and 6 and makes contact with the two busbars at regular intervals through cut away portions of the insulation 8 and 10 surrounding the busbars 4 and 6. Discrete lengths of resistance heating wire 12 are thus coupled across the busbars 4 and 6 for energisation therethrough. An electrically insulating sleeve 16 encases the insulated busbars and coiled wire 12. The sleeve 16 is translucent to enable the cut away portions of the insulation surrounding the busbars to be readily identified by eye. When the heating device is cut to a desired length it needs to be severed just beyond a cut away portion to avoid producing a long end section of the device where no heating takes place. <IMAGE>

Description

SPECIFICATION Heating Devices The present invention relates to heating devices.

The heating devices to be described are in the form of elongate fiexible heating strips and are particularly suitable for wrapping around pipes, for example, to prevent the contents thereof from freezing and for process heating of fluids in pipes.

According to the present invention there is provided a heating device comprising a pair of generally parallel extending electrically insulated busbars, the insulation enveloping said busbars being absent at regular intervals therealong so that each exposed portion of one busbar lies generally midway between adjacent exposed portions of the other busbar and vice versa, elongate heating means coiled around said electrically insulated busbars along the length thereof and making electrical contact with the busbars at each of said exposed portions thereof, and a translucent and electrically insulating sleeve enveloping said insulated busbars and coiled heating means.

An elongate flexible heating strip embodying the invention will now be described, by way of example with reference to the accompanying diagrammatic drawings in which: Figure 1 is a perspective view of a heating device with terminations at each end; Figure 2 is a front elevation of a length of heating element of the device of Figure 1 with parts cut away; Figures 3A to 3C are front elevations of one end of the device showing the end termination during different stages of formation; Figures 4A to 4C are front elevations of the other end of the device showing the end termination during different stages of formation; Figure 5 is an exploded perspective view of a junction where two heating elements are joined together; and Figure 6 is an exploded perspective view of a junction where four heating elements are joined together.

The heating devices to be described each employ an elongate heating element 2.

The heating element as shown in Figure 2 includes a pair of busbars 4 and 6 each encased in a respective electrically insulating sleeve 8 and 10. Each busbar 4 and 6 is in the form of a stranded wire alloy, while the sleeves 8 and 10 are of silicon rubber. The sleeves 8 and 10 are joined together to lie in side by side relationship.

At regular intervals along the pair of busbars for example at intervals of 0.9 m or 0.6 m a section of sleeve is removed to expose a portion 14 of the bare busbars. The two busbars thus present alternate exposed portions 14 along the length of the heating element 2. A heating wire 12, for example a chrome-ailoy resistance wire, is spirally wound about the two insulated-busbars along its length. At locations where bare busbars 4 and 6 are exposed the heating wire 1 2 makes physical contact with the exposed portions 14 of the busbars 4 and 6 so that when the two busbars are energised a current will flow through the section of heating wire located between the two exposed portions 14 of busbars.A translucent sheath 1 6 of silicon rubber envelopes the combination of the busbars 4 and 6, the sleeves 8 and 10 and the spirally wound resistance wire 12.

The heating element 2 is thus composed of a series of heating resistors connected end to end in which each resistor is individually energised from the two busbars 4 and 6.

The element 2 is supplied in relatively long lengths wound on a spool and is cut to length according to particular requirements. The opposite end sections of each cut length are then sealed for protection, one termination 1 8 being a closed end seal and the other termination 20 being to provide cold leads 22 and 24 for connecting the heating device to a power source (not shown).

It is important that the heating element should be active substantially over its whole length and for this reason the heating element should be cut at each end just beyond a portion 14.

The provision of a sheath 1 6 which is translucent enables this to be readily achieved since the portions 14 can be instantly identified by eye.

It will be appreciated that the lack of sleeve where a portion 14 exists will produce a depression in an adjacent section of the sheath 1 6 and so the portions 14 can also be identified by feel.

To form the termination 20 a gland 28 is fitted over the end portion of the sheath. A 50 mm section of the sheath 16 is removed from the end (see Figure 3A) and the exposed resistance wire is unwound and removed. A silicon adhesive is coated over the exposed sleeves and a small diameter sleeve or tube 30 of silicon rubber fitted over the sleeves 8 and 1 0. The inner diameter of the tube 30 is just sufficient to accommodate the sleeves 8 and 10 while the outer diameter is substantially equal to that of the sheath 1 6.

A silicon rubber adhesive is then applied over the tube 30 and an adjoining portion of the sheath and a large diameter sleeve or tube 32 of silicon rubber is fitted over the tube 30 and the adjoining portion of the sheath 1 6.

Once the adhesive has set, the gland is threaded into a junction arrangement (not shown) to complete a waterproof seal for the termination.

To form the termination 18, at the other end of the cut length, the heating element is severed a distance of about 50 mm from the last exposed portion 14.

The last 1 5 mm of sheath 16 is stripped off with a sharp knife (see Figure 4A). The exposed resistive wire is unwound and severed and the two sleeves 8 and 10 separated. One sleeve 10 and its associated bus 6 are cut close.to the end of the sheath 1 6 while the sleeve 8 and its associated bus 4 are cut to be about 5 mm longer (see Figure 4B).

A cup-shaped end seal mould 34 of silicon rubber is fitted with silicon adhesive and the end of the heating element is also covered with the same adhesive before it is inserted into the mould.

The end of the mould is pinched just prior to insertion of the heating element end to expel as much air as possible. The heating element end is inserted and the adhesive is allowed to set to complete the seal.

Figure 5 shows a junction for effecting a sealed joint between two heating devices.

As shown the junction cover which is of electrically insulating material comprises a planar upper member 40 and a lower member 42. The lower member is provided with a channel 44 having an enlarged central section 44A. A partition wall 46 extending in the longitudinal direction of the central section into two.

The stripped ends of the heating elements are brought together end to end and crimped so that one busbar of a first element is crimped to one bus of a second element and the other busbar of the first element is crimped to the other busbar of the second element. An RTV silicon sealant is then squeezed into the channel in the lower member and the two crimped joints inserted into the channel so that the joints lie on opposite sides of the partition wall 46.

The upper member 40 has its edges laid with RTV silicon sealant and the two members are then pressed together. A small piece of aluminium foil is wrapped around the coupled members 40 and 44 and the sealant allowed to set and cure.

Figure 6 shows a junction for sealing and electrically insulating two joints between four heating devices. In Figure 6 parts similar to those in Figure 5 are similarly referenced.

As shown the upper and lower members of the cover are both similar to the lower member 42 of the cover shown in Figure 5.

The stripped ends of four heating elements are brought together as two superimposed pairs lying end to end. The busbars are crimped together in sets of four. The two members 42 are filled with silicon rubber adhesive, have their mating surfaces coated with the'same and are brought together on opposite sides of the two crimped joints such that the two joints will be accommodated partially in each of the two ducts 44 but be separated by the two partition walls 46 when they are brought into engagement with one another.

The junction is then wrapped in aluminium foil and the sealant allowed to set.

It will be appreciated that instead of silicon rubber insulation, plastics, such as PVC or glass reinforced plastics, can be used, in single or multiple layers.

It will also be appreciated that once the opposite ends of the heating element have been sealed it is completely waterproof and can be used for both immersion heating as well as external heating.

The method of sealing described ensures that the seal will be ductile and will stand up to temperature variations ranging from -500C to 2000C. .

If appropriate, the heating device may include an earthing conductor of tinned copper or stainless steel.

Claims (10)

1. A heating device comprising a pair of generally parallel extending electrically insulated busbars, the insulation enveloping said busbars being absent at regular intervals therealong so that each exposed portion of one busbar lies generally midway between adjacent exposed portions of the other busbar and vice versa, elongate heating means coiled around said electrically insulated busbars along the length thereof and making electrical contact with the busbars at each of said exposed portions thereof, and a translucent and electrically insUlating sleeve enveloping said insulated busbars and coiled heating means.

2. A heating device according to Claim 2 wherein said busbars are severed at one end so as to terminate close to a said portion and means sealingly enveloping the busbars at said one end to render that end waterproof.

3. A heating device according to Claim 3 wherein said sealing means comprises a cupshaped member arranged to be filled with an electrically insulating sealant.

4. A heating device according to Claim 1 including sealing means for providing a waterproof seal at that end of the device through which the device is to be energised.

5. A heating device according to Claim 4 wherein said sealing means comprises a first electrically insulating sleeve having an inner diameter sized to accommodate the two insulated busbars and an outer diameter corresponding to that of the sheath, and a second electrically insulating sleeve having an inner diameter sized to accommodate both the first sleeve and the sheath, said first sleeve when fitted being adhesively secured to an end portion of the element bared of sheath and heating means and said second sleeve when fitted bridging the sheath and the first sleeve and being adhesively secured to both.

6. A heating device according to any preceding claim wherein said heating means comprises resistive heating wire.

7. A heating device according to any preceding claim wherein the material of the electrical insulation comprises a silicon rubber.

8. A pair of heating devices each according to Claim 1 and junction means sealing the two junctions between two busbars of the two devices, the junction means comprising a first electrically insulating member having a channel in one face thereof, said channel having an enlarged central region divided longitudinally of the channel by a partition, the said junctions being arranged to lie on opposite sides of the partition, a closure member for securing to said face to close said channel, and an electrically insulating sealant filling the unoccupied parts of the channel to seal the joints within the junction means.

9. Two pairs of heating devices each heating device being according to Claim 1 and junction means sealing the junctions between the busbars of the two pairs of heating devices, the heating devices of each pair being connected in parallel and the two pairs of heating devices being connected in series to form two junctions, each junction incorporating a respective one of the busbars of each of the four devices, the junction means comprising first and second electrically insulating members having mating faces, each mating face defining a mating channel having an enlarged central region containing a longitudinally extending partition to separate the two joints when located in said channel, and an electrically insulating sealant filling the unoccupied parts of the channels, when the two members are matingly secured together whereby to seal the joints within the junction means.

10. Cheating device substantially as hereinbefore described with reference to the accompanying drawings.