A method for generation of clean steam in a continuous digester system, where the continuous digester system comprises • a chip bin using clean steam for initial steaming of cellulose material fed to the chip bin in order to heat the cellulose material and reduce amount of air in the cellulose material flow; • a steaming vessel using dirty steam for a subsequent steaming of the cellulose material fed to the steaming vessel and where a stream of vent gases are withdrawn from the steaming vessel containing at least a part of the bound air in the cellulose material fed to steaming vessel; • slurrying means for slurrying the cellulose material that has been steamed to a desired concentration of solids in the slurry formed; • transfer means for transferring and pressurizing the slurry to the top of at least one treatment vessel, wherein at least one zone of one treatment vessel contains a cooking zone kept at full cooking temperature; • an extraction screen in or immediately following the cooking zone extracting at least spent cooking liquor kept at full cooking temperature, said full cooking temperature kept in the range 135 to 175°C, or extracting spent cooking liquor diluted with wash liquor added after the cooking zone in a countercurrent wash zone, the extracted diluted spent cooking liquor having a temperature at lowest of 120°C; • a series of flash tanks receiving the extracted liquor, that reduce the pressure of the extracted liquor and generates dirty flash steam from the extracted liquor; said method ch a ract e rised in that the dirty flash steam as well as the stream of vent gases from the steaming vessel is led to a common steam-to-steam converter, and where a clean steam is evaporated from clean water fed to the steam-to-steam converter by indirect heating from the dirty flash steam as well as the stream of vent gases from the steaming vessel. • a chip bin using clean steam for initial steaming of cellulose material fed to the chip bin in order to heat the cellulose material and reduce amount of air in the cellulose material flow; • a steaming vessel using dirty steam for a subsequent steaming of the cellulose material fed to the steaming vessel and where a stream of vent gases are withdrawn from the steaming vessel containing at least a part of the bound air in the cellulose material fed to steaming vessel; • slurrying means for slurrying the cellulose material that has been steamed to a desired concentration of solids in the slurry formed; • transfer means for transferring and pressurizing the slurry to the top of at least one treatment vessel, wherein at least one zone of one treatment vessel contains a cooking zone kept at full cooking temperature; • an extraction screen in or immediately following the cooking zone extracting at least spent cooking liquor kept at full cooking temperature, said full cooking temperature kept in the range 135 to 175°C, or extracting spent cooking liquor diluted with wash liquor added after the cooking zone in a countercurrent wash zone, the extracted diluted spent cooking liquor having a temperature at lowest of 120°C; • a series of flash tanks receiving the extracted liquor, that reduce the pressure of the extracted liquor and generates dirty flash steam from the extracted liquor;