| 摘要 |
<PICT:0815559/III/1> <PICT:0815559/III/2> A heater comprises an electrically and thermally conductive body having a heat transfer surface, e.g. a stainless steel tube 10, a heating resistor, e.g. a wire 12 wound thereon, the resistor being spaced from the body by an alumina coating 11 applied by burning a gas to form a high-temperature flame, introducing alpha alumina particles into the flame so as to sinter the particles and depositing a crystalline sintered adherent coating on the body. The tube 10 may be clamped on a pipe or materials to be heated are passed through the tube. The outer surface of the tube 10 is roughened, e.g. by etching or sand-blasting, and the alpha alumina particles are introduced into an oxy-hydrogen or oxy-acetylene flame. The particles injected may be of 200-550 mesh size per inch and may be aluminium acetate, bauxite or alpha alumina monohydrate, which form alpha alumina in the flame and may be mixed with 2-10 per cent of flux, preferably phosphates and 2-8 per cent of flame catalyst, e.g. magnesium fluoride. The coating may be 4 mils. thick and after winding on the wire 12, a second coating 11a may be applied. The coatings after deposition are largely gamma alumina. The assembly is spaced within a container 14, e.g. a steel tube, and insulating particles M, e.g. magnesia, are packed with vibration in the intervening space. The tube 14 is then swaged to reduce its diameter by 20 per cent, e.g. to 7/8 inch. The wire 12 is of nickel chromium and its ends 12a pass through insulating annular members 15 of reinforced packings spacing the ends of the tubes 10, 14. |
