Abstract: Indefinite strand yarn is steam and heat treated in an enclosure. The yarn travels in an indefinite length moving coil through a tunnel that extends through the enclosure, with steam being supplied to the enclosure and the exhaust pipes leading from adjacent the inlet and outlet of the tunnel to a blower for exhaust to the exterior of the enclosure. Periodically, the buildup of yarn filaments and partially solidified condensate on the interior surfaces are removed by spraying liquid solvent along such surfaces, preferably with nozzles that spray solid cones of liquid solvent axially down the various pipes and automatic timed controls for sequencing such spraying.