this invention relates to heat treatment of textile products . the apparatus comprises vertical furnaces each having two furnace ducts carrying blowing nozzles and suction nozzles , respectively . in the ducts of the last furnace , there are arranged movable horizontal partitions formed by the selective closure of baffles arranged at fixed levels and preferably constituted by butterfly - valves pivotally mounted on horizontal parallel axes . the invention is applicable to the heat treatment of continuous textile products , such as threads , ribbon or strip material . .