In a sizing process for a warp sheet of yarns, a drying chamber has an upper first region, substantially void of a drying medium, and a second region containing the drying medium. A transversable warp splitting device is provided in the chamber which is traversable from the first region, where a plurality of splitting bars can be inserted through the warp sheet so as to split it into sheets of lesser density, to a second region, where the warp sheet is acted upon by the drying medium.A cooling medium is provided between the first and second regions so as to prevent migration of the drying medium from the second region to the first region.The splitting device comprises a carriage driven for movement along a track which extends in a downwardly inclined direction from the first region to the second region.