steam that is required for complete steam pre treatment can in this way be reduced an upper level of chips is normally established at the top of the steam pre treatment vessel where the feed is controlled in such a manner that this level is established between a lowermost and an uppermost level a gas phase is established in the vessel between this upper chips level and the top of the vessel the steam pre treatment vessel shown in fig1 is a vessel in which impregnation of chips takes place in the lower part of the vessel as is shown in the drawing this may take place for example according to a technique that is sold by metso paper under the name impbin u2122 pressurised hot black liquor bl is preferably added to the vessel during this technique whereby the pressure on this hot black liquor is released and generates the principal fraction of the steam that is required for the steam pre treatment of the chips the steam that is expelled from the surface bl lev of the black liquor is indicated with bl st steam st may be added also at the lower parts of the steam pre treatment vessel through suitable outlet or addition nozzles well under the upper chips level that has been established where the amount of steam is regulated following detection of the temperature in the column of chips a measurement probe 32 is shown in the drawing which probe establishes a mean.