In the prior art ovens described above, the heating elements, typically electrical cartridge resistors, are installed in the half-clamps, so as not to be affected by the size changeover operations which involve the replacement of the reduction elements.

The reduction elements have a significant metal mass made of a material with a high level of heat conduction properties, typically an aluminium alloy, and they have one or more housings for respective pipes to be heated, with a number compatible with the nominal diameter of the pipe .

Each reduction element is stably fixed to the respective half-clamp by means of captive screws in such a way as to guarantee, at the coupling areas, the best contact between the respective surfaces of the half-clamp and the reduction element.