concentric layers with at least one layer arranged in each passage 3 4 fig3 illustrates the disposition of the heating elements more clearly the metallic cylinders 6 can be provided with flanges or other surface enlarging means which is also true of the heating elements 7 each individual ring 7 may define a separate heating element or several rings together may define a heating element by arranging it or them as an electrically closed circuit possibly by means of a shot circuiting device not shown the heating elements 7 may also be arranged as one or more helix helices or spiral s also with short curcuiting means not shown the heating elements 7 may be arranged concentrically around each other and or axially one after another the coil coils 2 may be one or more in number in the case of one coil normally a single phase electrical power supply is used and this can also be the case when several coils are used in the case of a plurality of coils these can be supplied with multi phase current e g with one phase per coil and the coils can be arranged axially after each other around the medium passageway or at the side of each other for example in the case of several heating devices where.