Heating of hydraulic digesters

A method of producing chemical pulp in an impregnation stage and a cooking stage, using a hydraulic digester, having a top separator, a level of chips and a liquid phase above the level of chips, comprising at least the following steps:

• a. entraining chips in liquor to produce a slurry having a first temperature,
• b. feeding the slurry of chips to the digester through the top separator,
• c. supplying the direct steam to a liquid phase between the level of chips and the top separator to heat the slurry to a second temperature for the impregnation stage, wherein the steam is fed by using at least one steam injector having a plurality of steam discharge openings;
• and
• d. heating the slurry to a third temperature and cooking the chips in a cooking zone in the hydraulic digester, and withdrawing the cooked pulp from a bottom of the hydraulic digester.