identical results from every treatment it is required that the duration of the steam treatment be constant said duration being determined from that moment where the spooled threads have reached the maximum temperature thereof to obtain such a constant duration there is provided the temperature measuring device 66 inside one spooled thread 69 and said device 66 controls according to the measured temperature said valves 45 47 and 15 the control should be such that the temperature measuring device 66 when the pre determined maximum temperature is reached inside the spooled thread will start a timer which after a specific duration for instance 12 minutes will close valves 45 and 47 and open valve 15 spooled threads 69 are thus treated with steam during a fixed time interval for instance about 12 minutes from that moment where the spooled threads have been heated enough for the treatment temperature to have reached the middle area of the spooled thread the closing of valves 45 and 47 and opening of valve 15 results in a depressurizing of the kier when no overpressure is present any more inside kier 1 valve 15 is closed and the following steps of the treatment are performed as shown in dotted lines in fig1 and 3 the cover of kier 1 may be connected directly to pipe 25 in such a case pipes 23 and 24 may be dispensed with in the embodiment as shown in fig1 and pipe 23 may be dispensed with in the embodiment as shown in fig3 in this case the overflow from kier 1 occurs through pipe 24 39 either to expansion tank 2 when valve 26 is open or to the.