vessel 3 via a return pipe 28 containing a pump 29 to pump the chips to the digester 4 with the aid of the separated liquid the return pipe 28 passes a heat exchanger 30 to heat the liquid passing it by the supply of high pressure steam via a pipe 31 the supply pipe 27 and return pipe 28 form a transfer circulation for the suspension of impregnated chips and boiling liquid a larger quantity of digesting liquor e is added to said transfer circulation in that the pipe 24 for the digesting liquor is connected to the return pipe 28 before pump 29 if desired a smaller quantity of the digesting liquor may be added to a circulation pipe 33 of the digester via a pipe 19 provided with a valve the digester is provided with a strainer 32 for circulation of liquid through pipe 33 with the aid of a pump 34 the liquid being heated in a heat exchanger 35 the pipe 33 comprises a central tube disposed in the centre of the digester with its orifice at the strainer 32 the digested chips are washed in countercurrent flow in the lower part of the digester using washing liquid supplied.