unit 23 for controlling the amount of washing water is started if the aim is to perform the washing in a pressurised state and consequently to affect the behaviour of the air in the system the control unit 23 is allowed to control valve 13 instead of valve 22 however valve 13 is not opened and washing is not started until the desired pressure has been reached in the washing chamber of the pulp mat 27 with the aid of the pressurised washing water starting to move the washing water pushes the fraction filtrate stored in holding pipe 1 a ahead of it through the pulp mat 27 with the result that the most concentrated fraction filtrate thereof displaces the most concentrated mother liquor from the mat directly into the wash filtrate tank 19 through pipe 20 the control unit 23 estimates how much filtrate is discharged into the wash filtrate tank and indicates when the desired volume is reached at this stage three way valve 14 is immediately turned into its second position so that the wash filtrate required in the continuous fraction wash is directed into fraction holding pipe 1 beginning to fill it after the fraction wash filtrate has run out the washing of the pulp mat continues uninterrupted using the regular washing water available until unit 23 for controlling the amount of washing water sends an impulse to close washing water valve 22 valve 22 is closed immediately as well as valves 11 and 13 the shifting of wash filtrate flow destination from wash filtrate collecting tank 19 to fraction pipe 1 using three way valve 14 is timed in the control unit in such a way that the smallest possible amount of fraction filtrate.