GB2059730A

GB2059730A - Deformable heating unit

Info

Publication number: GB2059730A
Application number: GB7932307A
Authority: GB
Original assignee: Cooperheat
Current assignee: Cooperheat
Priority date: 1979-09-18
Filing date: 1979-09-18
Publication date: 1981-04-23
Also published as: DE8025063U1

Abstract

A deformable heating unit comprises insulated resistance wire which is configured into a predetermined path, such as a serpentine path, maintained only by clips 5. Each clip is in the form of a strip having an open loop at each end to receive the insulated wire, the loops being closed by hand or by a compressive tool. A jib may be used in making a unit in a serpentine configuration, the jig including a series of ribs 13 with a movable end piece 10 at the end of each rib for adjusting the length of the configured unit. The heating units are hand deformable after manufacture to increase or decrease their length by a limited amount. A connector for reducing heat conduction from the heating unit to a conductor of a current supply, may be used, the end portion of the resistance wire being covered with silver solder and secured together with a copper conductor in a crimped ferrule. <IMAGE>

Description

SPECIFICATION Deformable heating unit for use in pre-heat s post-heat treatment of welded joints This invention relates to a deformable heating unit which is particularly useful in pre-heat and post-heat treatment of welded joints but which can be used for other types of heat treatment.

Heating units are known wherein an insulated resistance wire is configured or is doubled into a loop and then configured into a serpentine path, the serpentine configuration being maintained by interwoven ribbons and/ or wires of heat resistant material. For example, the length of the insulated resistance wire in each fold of the serpentine configuration forms a warp over and under which metal ribbons or wires pass as a weft. Additional metal wires, also forming a warp, reinforce the structure between the folds of the serpentine configuration and also form woven edges at each side of the heating unit. The finished unit is deformable and it can be used with a number of other units to surround, for example, a welded seam between two circular pipe sections.Although the structure and versatility of these units enables them to be used for the heat treatment of articles of different shapes, they have two main disadvantages. First they are expensive to manufacture since they must be either hand woven, or manufactured on a special machine capable of handling metallic ribbons and wire. Second, the overall dimension of the unit is fixed with regard to its length and breadth. Thus, when placing a plurality of such units, side by side, around a circumferential weld, either the size of the unit must be selected so that no gap is left in the circumferential heating structure, or an overlap is necessary between the first and last unit to avoid leaving such a gap.

The present invention seeks to overcome this problem and at the same time permit greater versatility in the arrangement of flexible heating units.

According to the invention, a deformable heating unit comprises deformable insulated resistance wire which is configured into a predetermined path having adjacent sections supported by clips.

The path may be serpentine, spiral, or other shapes provides that adjacent sections are not separated by more than a distance consistent with the required strength of the finished unit.

The insulated wire may be used as a single length which is configured into the predetermined path, or it may be doubled before it is configured into the predetermined path whereby the ends of the wire are situated at the same side of the unit. The clips may each be in the form of a strip of deformable material, such as stainless steel, with ends which are turned over to define loops or collars in which a respective section of the insulated wire is received. The material and construction of these clips may be such that the clips are removable after assembly and use of the structure, to permit the insulated wire to be used on its own or to be configured into a different configuration secured by further, or the same clips to provide a heating unit of a different required shape.

The invention also provides a method of manufacturing deformable heating unit comprising the steps of: (a) confining deformable insulated resistance wire into a predetermined path with adjacent sections, and (b) attaching clips to said sections for maintaining said path.

Suitably, the step of configuring the insulated wire into a predetermined path is carried out with the aid of a jig. Preferably, the jig is adjustable to enable adjustment of the size and shape of the finished heating unit. For example, the jig may comprise a base board on which plurality of members are removably located in spaced positions to define a support for the insulated wire, configured into the predetermined path, before the adjacent sections of the configuration are secured by the clips. The removable members serve at least as posts around which the insulated wire is led to form, for example, loops. The removable members may include ribs, or fixed ribs may be provided on the base board to cooperate with removable members in the form of posts. In either case, adjacent ribs define grooves for receiving sections of the insulated wire.The ribs preferably include recesses to accommodate the clips. The removable members and clips are preferably provided with holes so that they can be located on corresponding upstanding pins fixed to the base board.

Suitably, each clip is made of heat resistant deformable material and is in the form of a strip having its ends deformed in opposite directions to form an open loop (i.e. like a C and a reversed C). The centre part of the strip has a hole for receiving a respective pin on the base board. The insulated resistance wire is passed into each open loop which is then closed, for example, either by hand, or by a compressive tool such as a pair of pliers.

A suitable jig for producing a heating unit in which the insulated wire has a serpentine configuration comprises the following parts. A base board has a plurality of pins for locating a plurality of end pieces arranged in top and bottom rows adjacent opposite sides of the base board. These end pieces act as posts for forming the loops at the sides of the serpentine configuration. In each row, the pins are arranged in columns so that the positions of the end pieces can be adjusted to vary the width of the finished heating unit (the width extending transversely of the loops in the serpentine configuration). A series of ribs are either fixed or removably attached to the base board so that they extend alternately from the opposite sides adjacent the first and second rows of the end pieces.These ribs define grooves which receive the up and down sections of the insulated wire in the serpentine configuration. The ribs may be removably located on the base board so that their positions, in the row direction, can be varied to adjust the length of the finished heating unit.

This would also enable the same jig to be used for configuring either a single length, or a looped length of insulating wire into a serpentine path. The advantage of using a looped or double length of the insulated wire is that its end portion extends away from the same end of the finished unit which is more convenient for making subsequent electrical connections. The ribs include recesses for accommodating the clips and a pin is upstanding in each recess for locating the respective clip.

Preferably, the insulated wire used for making these heating units is protected by a covering of heat resistant braided wire, the construction being substantially without resilience to form a self-supporting structure such as a coil or loop. The clips are required to ensure that the configuration is maintained, especially when the units are handled and placed around structures to be heat treated and subsequently heated. Further, since the wire itself is deformable and it is held in spaced positions by clips, this enables the shape and/or dimensions of the unit to be changed within certain limits so as to fit against a structure to be heat treated.For example, with a serpentine configuration, the deformability of the heating unit enables its length to be changed by about + 10% (the length being in a direction substantially perpendicular to the up and down sections of the serpentine path). The heating unit can also be curved or folded with respect to its length or width. This deformation can be achieved by hand thereby facilitating a change in shape or dimensions when the heating units are used on site.

The advantages of the invention are that the heating units are far less expensive to produce than the woven heating units since no hand weaving or special weaving machine is required and the heating unit may also be readily made up into a required configuration and/or the clips can be removed subsequently to enable the heating wire to be used on its own, for example, by wrapping it around the pipe in the fashion of a coil and/or configured and secured by the same or further clips into a unit of a different shape and/or dimensions. Thus, the heating unit can be easily manufactured as complete assemblies and sold as purpose-made articles, and/or the insulated resistance wire and clips and optionally an adjustable jig can be provided to enable a user quickly to assemble a unit of the required dimensions and shape.

Furthermore, the deformability of the construction enables the shape and dimensions of the finished heating units to be adjusted, within limits, by hand to allow for differences in the dimensions and shape of structures to be heated by an assembly or a plurality of heating units.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a plan view of a heating unit according to the invention and including a folded length of insulating wire configured into a serpentine path; Figure 2 is a plan view of a jig for producing a heating unit in accordance with the invention, the unit having a single length of insulating wire configured into a serpentine path.

Figures 3 and 4 are respectively a side elevation and plan view of a clip which is one of several used in the structure of Fig. 2.

Figure 3 is a side elevation, partly in section, of the jig and heater unit shown in Fig.

2.

Figure 6 is a schematic view of a part of a heating unit which has been deformed, by hand, Figure 7 is a schematic plan view of a further heating unit in which the resistance wire, used to the folded loop, has been configured into a spiral, and Figure 8 is a schematic cross section of a preferred connector for reducing heat conduction.

Referring to Fig. 1, this illustrates a heating unit as a complete assembly. It comprises a folded length of insulated resistance wire which has been configured into a serpentine configuration 2. The centre 3 of the folded length of wire 1 forms a corner of the configuration or mat 2 and the free end of the wire 1 extends away from an opposite side of the configuration 2 towards respective connectors 4a, 4b. The serpentine configuration has a series of pairs of loops 3a on each side of the configuration 2. Sections 3b which extend between the loops 3a are secured by a plurality of clips 5. The clips secure adjacent sections of the wire 1 as shown in the drawing and are staggered or offset so that the serpentine configuration is maintained. Some of these clips 5a secure adjacent sections of the end portions of the wire 1 to provide a neat terminal end for the heating unit.

The insulated wire 1 generally comprises a central core of resistance wire, which is nonresiliently deformable, and which is surrounded by an insulating jacket. The insulation is protected by an outer cover of braided heat resistant wire which is non-resiliently deformable. A suitable construction may com prise, for example, a multi-strand nickel chrome wire core having a resistance of 0.084 ohms per ft. with a diameter of 0.0226 inches (approx. 0.0574 cm) overlaid by six strands of 0.0206 inch (about 0.0523 cm) diameter nickel chrome wire wound clockwise and then overlaid by twelve strands of 0.0206 in (about 0.0523 cm) diameter nickel chrome wire wound anti-clockwise. The made-up core is then annealed in a controlled atmosphere furnace to reduce springiness.

The core wire is then insulated with a proprietary material which is basically a chemical leached glass fibre in the form of cordage.

The cordage is braided into the conductor usually in two layers. The core is then finished off by over-braiding with sixten groups of 0.0076 inch (about 0.0193 cm) diameter nickel chrome wire with eight wires to a group.

Connectors 4a, 4b are preferably of a special construction to reduce local heating between the ends of wire 1 and respective conductors. The conductors are connected to known terminal pins (not shown) received in sockets (not shown) connected to a conventional power source, such as a welding current generator (not shown). More particularly, the connectors 4a, 4b have the construction shown in Fig. 8. The insulated reistance wire 1 has a core 20 which is coated over, for example, the last 3 inches (about 8 cm.) with silver solder 21. Insulation 22 and protective cover 23 are cut back from the end of the wire 1 and secured by a stainless steel ferrule 24. The bared and coated end of the wire 1 extends through a stepped ceramic insulating coliar 25 which fits the end of a ceramic insulating sleeve 26.The terminal end of the wire 1 is received in a bore 27a in a heavy mild steel crimpable ferrule 27. Ferrule 27 has another bore 27b which receives the end of a conductor 28 such as a copper welding cable of 1 6 mm diameter. The conductor 28 has an insulating cover 29 and passes through a mild steel plated retainer clip 30.

The ferrules 24 and 27 and the clip 30 are crimped to secure the parts passing therethrough, the ferrule 24 and clip 30 being spaced so as to accommodate the ceramic collar 25 and sleeve 26 which are thus held together.

The connector described and illustrated with respect to Fig. 8 provides a form of "cold junction" which reduces local heating between wire 1 and the copper conductor 28.

This overcomes a typical problem which arises when connecting a heating unit to a copper termination cable. For example, when a nickel-chrome core is merely crimped with a copper conductor, a heating effect (proportional to the square of the current and the resistance of the crimp) takes place. Since copper is subject to oxidation at the temperatures experienced in use and also becomes brittle, such a connection usually leads to a breakage of the copper conductor at or near the crimp. Whilst the temperature at the crimp could be reduced by using a large, heavy walled crimping ferrule, together with a heavy large cross-section termination cable, thus providing a heat sink to reduce the junction temperature, this would only give a marginal improvement and would add the problem of increasing the weight and reducing the flexibility of the unit.Thus, the solution lies in reducing the resistivity (compared with, for example, a nickel chrome core) of the terminal end of the insulating wire 1 whilst still retaining a high heat tolerance. A pure nickel core could be used but nickel has the disadvantage of being very prone to attach by sulphur-based contaminants at elevated levels. This is a problem where heating units are used in petrochemical plants. Therefore, it is preferred to reduce the electrical resistance of the terminal end of wire 1 by the application of silver solder which impregnates and coats the terminal end of a multistrand resistance wire. The connector described and illustrated in Fig. 8 can be applied to heating units or to heating wires other than those described in detail in the specification.

A plurality of the configurations or mats 2, shown in Fig. 1, can be joined or assembled side by side so as to form, for example, a band which encircles a pair of circular welded pipe sections. The band surrounds the weld and is itself insulated, for example by ceramic fibre blankets to retain the heat. The heating units may be connected in series or parallel, depending on the output of the generator, and so that they attain a required temperature for heat treating or annealing the weld.

Referring now to Fig. 2-5, a method will be described for producing a heating unit having a single length of insulated wire in a serpentine configuration. The same method may be applied, in principle, to make the heating unit shown in Fig. 1 having a folded length of resistance wire.

The method involves the use of adjustable jig 7, as shown in Fig. 2, comprising a base board 8 on which a plurality of upstanding pins 9 are provided for locating removable end pieces 10 and clips 5. The end pieces are each provided with two holes 11 to receive the respective pins 9, two pins being provided to prevent the respective end piece from pivoting on the base board 8. Each end piece is in the form of a block having a convex face 1 2 around which the resistance wire 1 is led to form respective loops 3a, 3b on each side of the serpentine configuration. The end pieces are regularly spaced in a top row 1 0a and a bottom row lOb but are offset to accommodate the loops 3, 3b.The pins 9 adjacent each row 1 0a, 1 0b are arranged in columns so that the positions of the end pieces 10 can be adjusted to vary the width of the finished heating units.

A series of ribs 13, extending alternately from offset sides of the jig 7 (adjacent rows 10a, lOb) are also fixed to the base board.

The ribs are regularly spaced by the required width which, in this case, is slightly greater than the diameter of the circular section heating wire 1. The ribs extend toward the respective end pieces in rows 1 0a, 1 0b and each rib has a series of recesses 14 along its length.

As best seen in Fig. 5, one of the pins 9 is also provided in each recess of the ribs 1 3 to locate the respective clips 5. The clips 5 are shown in more detail in Figs. 3 and 4 and illustrate a hole 1 5 for receiving the respective pin 9. Several recesses are provided along the length of each rib so that the positions of the clips 5 can be varied with regard to varying the dimensions of the heating unit by adjusting the positions of the end pieces 10 in rows 10a, lOb.

Thus, adjacent ribs 1 3 form grooves in which the up and down sections of the insulated resistance wire 1 are received and the end pieces 10 act as posts around which the loops 3a, 3b are formed.

Referring to Figs. 3 and 4, each clip 5 is made of stainless steel and is in the form of a strip configured into a centre piece 1 7 with an open loop of collar 18, 1 9 on each side of the centre piece 1 7. After introducing the insulated resistance wire into each loop 18, 19, they are closed as indicated by the broken lines. The clips may be closed either by hand or with the aid of a compression tool such as a pair of pliers. After closing each of the clips in the jig, the heating unit is removed. The terminal connectors 4a, 4b may be applied to the end of the wire 1 either before, or after securing the clips in the jig. The construction and method of applying the terminal connections is known in the art and will therefore not be described.

The jig described above can be modified for use with a looped length of insulated resistance wire 1 for producing the heating unit shown in Fig. 1. In this case, the spacing of the ribs 1 3 is increased to accommodate each section of the doubled loop. The ribs 1 3 could be removably attached to the base board 8 to enable adjustment of the lateral spacing.

Either the heating unit shown in Fig. 1, or the unit made with the jig described with reference to Fig. 2, can be further deformed by hand to increase or decrease its length by about 10%. This is due to using insulated resistance wire which can be deformed per se and securing the sections of the wire at spaced positions by the clips 5. Fig. 6 schematically illustrates the deformation of a heating unit by expansion, i.e. by holding each end and pulling outwardly. The intermediate sections are pulled away from one another giving an increase in the length of the heating unit. Conversely, the length of the unit may be decreased by pushing the sides inwardly towards one another whereby the intermediate sections would bow towards one another. This type of expansion and contraction is impossible with the woven heating unit of the prior art.The possibility of expanding or contracting the units by hand is a most useful advantage, because the units can be quickly modified on site, to account for small changes in the overall dimensions of the structures which are to be heat treated. A plurality of the heating units may not exactly fit around the sides of a particular structure and thus one or more units can be expanded or extracted to take up the gap, or to prevent an overlap.

Even when the structures are, for example, pipe sections which change in diameter at the location of a weld, the heating units of the present invention can be manipulated to fit by stretching one side and compressing the other. It is important in the post-heat treatment of welds to apply a uniform heating structure without gaps, which can cause harmful thermal gradients in the work piece, and without overlaps which can cause local hotspots and thus reduce heater unit life.

Fig. 7 schematically illustrates how a heating unit can be produced according to another embodiment of the invention, wherein the wire 1 is first doubled and then wound into a spiral which is maintained by clips 5 securing adjacent sections.

Claims

1. A deformable heating unit comprising deformable insulated resistance wire which is configured into a predetermined path having adjacent sections supported by clips.

2. A heating unit according to claim 1 wherein the path is serpentine.

3. A heating unit according to claim 1 wherein the path is spiral.

4. A heating unit according to any one of the preceding claims wherein the insulated resistance wire is used as a single length configured into said predetermined path.

5. A heating unit according to any one of claims 1-3 wherein the insulated wire is doubled before configuration into the predetermined path whereby the ends of wire are situated at the same side of the unit.

6. A heating unit according to any one of the preceding claims wherein each clip is in the form of a strip of deformable material, the strip having ends turned over to define open loops which receive the insulated wire.

7. A deformable heating unit substantially as herein described with reference to Fig. 1 of the accompanying drawings.

8. A method of manufacturing a deformable heating unit comprising the steps of: (a) configuring deformable insulated resistance wire into a predetermined path with adjacent sections, and (b) attaching clips to said sections for main taining said path.

9. A method according to claim 8 in which the insulated resistance wire is configured into a predetermined path with the add of a jig.

10. A method according to claim 9 wherein the jig is adapted for adjustment by the provision of removable members, said members being located in spaced positions to define a support for the insulated wire to be configured into said predetermined path.

11. A method according to claim 10 in which the wire is configured into a serpentine path, the jig comprising a base board having a series of spaced ribs and the removable members being in the form of blocks which are adjustably positioned at the end of each rib, the adjacent ribs defining grooves into which the insulated wire is introduced and the end pieces acting as posts about which the wire is turned to form loops in the serpentine path.

1 2. A method according to claim 11 wherein the ribs contain a series of recesses into which the clips are placed, a pin extending in each recess on which the respective clip is located.

1 3. A method according to claim 11 or 1 2 wherein the lateral position of the ribs is adjustable on the base board.

14. A method of manufacturing a defor mable heating unit substantially as herein described with reference to Figs. 2 and 5 of the accompanying drawings.

1 5. A deformable heating unit made according to the method of any one of claims 8-14.

1 6. A deformable heating unit according to any one of claims 1-7 or 1 5 wherein at least one end portion of the insulated resistance wire is covered with material of a lower resistivity than said resistance wire and said end portion is electrically connected to a conductor.

17. A deformable heating unit according to claim 1 6 wherein said end portion is covered with silver solder.

1 8. A deformable heating unit according to claim 1 7 wherein said end portion and said conductor are both secured in a ferrule.

1 9. A deformable heating unit according to any one of claims 16-18 wherein the end portions of the resistance wire are connected to respective conductors substantially as herein described with reference to the accompanying drawings.

CLAIMS (9 Dec 1980)

20. A deformable heating unit comprising a deformable insulated resistance wire which is configured into a predetermined path, said wire being maintained in said path by means of independent clips made of material which is deformable by hand, or by a hand tool, to facilitate assembly of the heating unit with the required dimensions and configuration, said clips enabling a limited change in the shape or dimensions of said unit by deformation of the unit to adjust the relative spacing between adjacent sections of said path.

21. A heating unit according to claim 20 wherein said insulated resistance wire is configured into a serpentine path, said clips being offset from one another, in the directions of transverse sections of the serpentine path, to enable the length of the heating unit, as measured across said transverse sections, to be altered by pulling apart, or pushing together said transverse sections of the heating unit.

22. A heating unit according to claim 20 or 21 wherein each clip is in the form of a strip of said deformable material, the strip having ends turned over to define open loops which receive respective portions of said insulated resistance wire.

23. A heating unit according to any one of claims 20-22 in which said wire is in the form of a doubled loop before it is configured into said serpentine configuration, said clips also being used to secure adjacent end sections of said insulated resistance wire.

24. A method of manufacturing a deformable heating unit comprising the steps of: (a) configuring deformable insulated resistance wire into a predetermined path with adjacent sections, and (b) attaching independent clips to said sections for maintaining said path, said clips being made of material which is deformable by hand, or by a hand tool, to facilitate assembly of the heating unit with the required dimensions and configuration.