Method and Apparatus for Converting a Fluid Tracing System Into an Electrical Tracing System

This invention relates to a method and apparatus for converting a fluid tracing system into an electrical tracing system. The invention will be discussed primarily with reference to a typical fluid tracing system, namely, a steam tracing system. Any differences between the illustrative steam tracing system and other fluid tracing systems will be understood by those skilled in the art, having regard to their own knowledge and the disclosure herein.

Steam tracing systems are well known and typically include a substrate to be heated, an interconnecting system of steam tracing tubes adjacent the substrate, condensate return lines and thermal insulation surrounding the substrate and steam tracing tubes.

In converting such a steam tracing system into an electrical tracing system, it is possible to remove the existing thermal insulation and then the steam tracing tubes, in turn followed by securing an electrical tracing system adjacent to the substrate and fitting this system with new insulation. It has been found, however, that this is a costly as well as a wasteful conversion method.

The present invention provides a novel method and apparatus for converting a fluid tracing system, e.g. a steam tracing system, into an electrical tracing system. The invention significantly reduces cost and avoids waste.

In one aspect, the invention provides a method of converting a thermally insulated steam heat-tracing system for a substrate into an electrical heat-tracing system for the substrate, said steam heat tracing system comprising a substrate, a plurality of inter-connected elongate tubes adjacent to and in thermal contact with the substrate so as to permit steam to be passed through the tubes in order to warm the substrate, and thermal insulation surrounding the elongate tubes and the substrate, which method comprises

(1) at spaced-apart locations, removing the thermal insulation and opening up the plurality of interconnected tubes so that elongate electrical heating elements can be inserted into said tubes,

(2) inserting elongate electrical heating elements into said tubes, and

(3) connecting said elongate electrical heating elements to a power supply in order to heat the substrate.

In another aspect the invention provides an apparatus comprising

(a) a substrate to be heated;

(b) a plurality of interconnected elongate tubes which are adjacent to and in thermal contact with said substrate, at least some of said tubes having once formed part of a former heating system for said substrate, in which system a heated fluid was passed through the tubes and thermal insulation surrounded the tubes and the substrate;

(c) an elongate electrical heating element which lies within and extends along said elongate tubes and which can be connected to a power supply to provide an elongate electrical heater; and

(d) thermal insulation which surrounds the elongate tubes and the substrate, at least part of said thermal insulation being the same as the thermal insulation in said former heating system.

The invention is illustrated in the accompanying drawing in which

FIG. 1 is a schematic diagram of the method and apparatus of the invention and

FIGS. 2, 3 and 4 are schematic drawings of the Examples which follow.

As previously noted, the invention makes use of an elongate electrical heating element which can be connected to provide an elongate electrical heater. The heater is preferably self-regulating and may be, for example, a parallel heater.

Preferably, the heating element comprises at least two elongate conductors and a plurality of heating elements connected in parallel with each other between the conductors. Preferably, the heating element comprises a conductive polymer composition which exhibits PTC behavior. A preferred heating element comprises at least two elongate, spaced-apart conductors and an elongate resistive heating strip which is composed of a PTC conductive polymer and is wrapped around the conductors. For further details of such a heating element, reference may be made to U.S. Pat. No. 4,459,473, incorporated by reference herein. Many of the self-regulating heaters disclosed in U.S. Pat. No. 4,459,473 have a generally "flat" sided geometric configuration, to ensure adequate heat transfer from the heater to a substrate to be heated. For purposes of the present invention, however, we have found that it is advantageous to use a heater with a generally circular geometric configuration of radius 0.125 to 0.5 inch, for ease of installation in the elongate tubes and to maximize the power output of the heater. This makes it preferable to modify the products and procedures described in U.S. Pat. No. 4,459,473 in the following ways. First, the two elongate, spaced-apart conductors preferably are now entirely coated with a layer of conductive material e.g., a low resistivity substantially ZTC conductive polymer composition, before being contacted by the heating strip or wrapping. In the preferred procedure disclosed in U.S. Pat. No. 4,459,473, by contrast, only a portion of the conductor is coated, and this coating is done after the conductors are contacted by the heating strip. Second, the coating and wrapping steps are now carried out in a single stage, in contrast to a two stage process; this provides substantial cost savings and improvement in production rates. Third, to minimize twisting and rotation of the circular heater, annealing is now done at substantially higher temperatures (than for the flat geometry), e.g., at 400.degree. F. to 600.degree. F., as compared to the previous range of 400.degree. F. to 490.degree. F. The heater is connected to a suitable power source.

In another embodiment, the heater may comprise a solid insulated ferromagnetic wire e.g., a 14 AWG solid nickel wire; a ferromagnetic cladding over a paramagnetic wire (e.g., nickel clad copper wire); or a distributed one turn transformer with a shorted secondary.

The heater is connected to a suitable power source, which may be for example a conventional line voltage, e.g., of 110, 220, 440 or 600 volts AC, or a high frequency constant current supply. When a ferromagnetic heater is used, it is preferably adapted to operate when supplied with a current between 5 to 40 amps over a frequency range of 10K kz to 100K kz. Note, in this regard, that the steam tracing tubes, typically copper, provide a means to overcome any high frequency electromagnetic interference of the heater.

To complete an electrical circuit, there may be provided a return conductor e.g., a copper wire, which can be an integral part of the heater or a separate wire outside of the heater. Alternatively the return conductor can be provided by the steam tracing tubing or the condensate return line. The return conductor may also be an elongate heating element.

Since conventional steam tracing tubes typically have a 0.25 to 2 inch inner diameter, it is desirable in all cases to provide a heater with dimensions proportioned to fit inside the tubes with relative ease. It is also desirable to provide means for enhancing heat transfer from the heater to the tube, for example, by providing oil in the tube.

A particularly suitable heat transfer medium is a gel which lies between the heating element and the tube surrounding the element. Gel has an advantage over oil since it will not leak from the steam tracing tube. At the same time, it provides substantially the same high heat transfer characteristics as oil.

Preferably, the gel comprises a particulate thermally conductive, electrically insulated filler dispersed therein. Preferably, the gel is a polymeric material, for example, a polysiloxane, a polyurethane or a butyl rubber, which is introduced into the tube in the form of liquid precursors which gel in situ in the tube, e.g., with the aid of heat generated by powering the heating element.

A suitable gel may be prepared by using a two-component room temperature vulcanizing silicone rubber compound, for example, one available under the Trademark General Electric two part silicone material RTV6157A, and, a heat transfer oil, for example, one available under the Trademark General Electric SE-96-50, cured for 30 minutes at 150.degree. F. For example, 5 grams of the silicone material can be mixed with 40 grams of the heat transfer oil and 5 grams of a catalyst, for example, a catalyst available under the Trademark General Electric CAT6157B.

Attention is now directed to FIG. 1 which provides a schematic diagram of the method and apparatus of the invention. A steam tracing system 10 is shown that includes a substrate 12 to be heated, an interconnecting system of steam tracing tubes 14 adjacent the substrate 12, and thermal insulation 16 surrounding the substrate 12. The steam tracing system 10 is converted into an electrical tracing system by opening up the steam tracing system at selected points, for example location 18, so that an elongate resistive heating element 20 can be inserted into the steam tracing tubes 14. The heating element 20 is installed throughout the steam tracing system by using, for example, a conventional "BLO-Rope" gun technique, to pull the heating element 20 through the steam tracing tubes 14.

A steam tracing system, converted into an electrical tracing system as illustrated in FIG. 2, included a substrate 22, a coaxial heating cable 24 with a 18 AWG stranded copper conductor 26 insulated with a polyolefin jacket 28, a tinned copper braid 30 used as a return conductor and an outer insulation 32 of polyolefin. The cable was inserted into a two foot section of a steam tracing system 34 comprising copper tubing. A power supply (not shown) was connected to the copper conductor and the braid and a constant current of 25 amps was passed through the cable. The following data was compiled.

A steam tracing system, converted into an electrical tracing system as illustrated in FIG. 3, included a substrate 36, a self-regulating heating cable 38 comprising 14 AWG solid nickel clad copper wire 40 with glass fiber insulation 42. A return conductor 44 comprised 14 AWB stranded copper wire 46 with glass fiber insulation 48. Both cables were inserted in a two foot section of a steam tracing system 50 comprising copper tubing, and one end of both cables was connected to a power supply (not shown) and the other end connected to each other. The power supply was a constant current, high frequency power supply and a current of 10 amps at 20 KHz was passed through the cable. The following data was compiled.

A steam tracing system converted into an electrical tracing system as illustrated in cross-section in FIG. 4, included a self-regulating heater 52 of the type disclosed in U.S. Pat. No. 4,459,473, but modified, in accordance with this disclosure, so that it had a circular geometric configuration for easy installation into a tube 54. The system included a gel 56 which lay between the heater 52 and the tube 54 surrounding the heater 52. A layer of thermal cladding 58 surrounded the tube 54.