parallel to the yarn guides 7 and form a yarn guide track 6 2 likewise the yarn guides 8 are arranged with their guide edges 9 alternatingly on opposite sides in spaced relationship with one another so that they form a zigzag yarn guide track 6 2 in the illustrated embodiment the yarn guide tracks 6 1 and 6 2 are formed by a different number of yarn guides 7 and 8 in the yarn guide track 6 2 the inlet region of the heater comprises a plurality of yarn guides 8 arranged one after another at short intervals with no yarn guides in the center region in comparison therewith the yarn guide track 6 1 is formed by a number of yarn guides 7 which are arranged offset from one another in an evenly spaced relationship 0028 fig2 is an end view of the yarn guides 7 and 8 the yarn guides 7 and 8 each have an l shape yarn guide edge 9 the l shaped yarn guide edge 9 is used to achieve a guidance of the yarn that is defined by its spacings from the side walls 3 and 4 as well as from the bottom wall 5 the yarn guide edges 9 of the yarn guides 7 and 8.