Abstract: A drawn gear-crimped polyester yarn with latent bulk is provided, the amount and nature of the bulk being such that the yarn has an initial crimp as defined of at least 1.5%, preferably of above 2%, and a mechanical crimp stability as defined of above 0%. The crimped polyester yarn may be produced by heating a drawable polyester yarn having a birefringence in the range 32.times.10.sup.-3 to 125.times.10.sup.-3 inclusive, preferably 35.times.10.sup.-3 to 125.times.10.sup.-3 inclusive, crimping the yarn by guiding it between the intermeshing teeth of a set of toothed wheels such that the yarn is caused to follow a sharply zig-zag path, the toothed wheels being rotated at a sufficient speed such that the yarn is drawn by the tension so imparted to the yarn by the toothed wheels and subsequently forwarding the crimped yarn from the teethed wheels under a controlled tension within the range 0.15 to 0.50 g per decitex inclusive, based on the decitex of the drawn polyester yarn.

Abstract: Process and apparatus for transferring heat between an advancing filamentary yarn and a fluid in which the yarn is passed through a vortex in the fluid substantially along or parallel to the longitudinal axis of the vortex.