c and a flow of 1 5 tonne hour bl 3 132 u00b0 c and a flow of 1 5 tonne hour rec 30 u00b0 c and a flow of 0 96 tonne tonne i e 0 96 tonne fluid per tonne of chips a temperature of the mixture of approximately 117 u00b0 c is obtained under these conditions which together with the exothermic reaction with the black liquor which corresponds to a temperature rise of approximately 5 u00b0 c ensures a final temperature of approximately 122 u00b0 c of the chips when fed out from the impregnation vessel at this level of the flow in the counterflow zone z 1 which preferably lies within the interval 50 150 of the flow of chips calculated as a weight percentage i e that 0 50 1 50 tonnes of fluid per tonne of chips is withdrawn at the flow rec a first heating of the chips is obtained in direct heat exchange between the chips and the counterflow of impregnation fluid which means that the temperature of the impregnation fluid is gradually reduced up through the zone z 1 from its value of 105 u00b0 c down to 30 u00b0 c by adjusting the withdrawal flow or by adjusting the cooling in the heat exchanger 20 the withdrawal temperature can be maintained essentially constant at such a low value that the impregnation fluid does not cause evaporation.