shown in full lines in fig2 to a lower limit position shown in broken lines at 24a in fig2 a pair of suitable fluid actuators 25 or other positioning mechanisms are provided for this purpose in a preferred form of the invention the upper guide roller 24 is mounted at each end in movable bearings 26 which are slidably guided in channels 27 mounted on opposite side walls 28 of the steam chamber thus as the actuators 25 are extended the bearings 26 and the upper guide roller 24 are supported and guided laterally by the channel 27 at the uppermost position of the roller 24 the bearings 26 which are formed with tapered upper surfaces are received in a seat 29 of inverted v shaped configuration to lock the bearings rigidly in position the retractability of the upper guide roller 24 is particularly desirable for initial threading of the processing line with a new length of fabric initially the guide roller 24 is retracted to its lowermost position indicated in fig2 in which it lies below a plane defined by the bottom surfaces of the entry and exit side guide rollers 22 23 thus in the initial threading operation the pre processed fabric 10 can be led horizontally underneath the guide roller 22 over the top of the guide roller 24 in its retracted position and underneath.