Contactless heating of moving wires
Paulette S Sidky, M Gwyn Hocking
A novel method is described for uniform radio frequency (RF) heating of static or moving wires at atmospheric pressure, over long lengths, without the use of contacts or brushes. Conventional induction heating of wires is not possible because of the large annular coil gap and the RF skin effect if the wire diameter is comparable to the skin thickness. The method described involves placing a rod, of length equal to the heated zone required, parallel to the wire to be heated and about 2cm from it, which allows the wire to be enclosed in a glass tube containing a protective atmosphere if required. The rod is connected to a 13.56 MHz RF power source. Heated wire lengths tested ranged from 1 mm to 70 cm, and no indication was found of an upper limit of heated length. No special tuned length of wire is required and there is no upper limit to the temperature obtainable. The temperature along the wire can be either uniform or a temperature gradient can be set up by bending the RF rod closer to or further from the wire in a local region. A simple RF safety screen of 2 cm wire mesh size was found to prevent RF emission to the operator and environment. The method is of interest in wire processing and in chemical vapour deposition of coatings on wires, including insulating coatings, etc. The alternative method of resistive heating with physical contacts may cause surface damage and sparking and does not allow optional controlled temperature gradients along the wire, nor heating at all if there is an insulating coating.