Chip Steaming

The continuous cooking process starts when the chips enter a steam environment.

The target of steaming is the elimination of air from the chips to a maximum

extent. As the chips are warmed up, the air is positively displaced from inside

them by the increasing partial pressure of wood moisture and by its own increasing

volume. The residual air removal must occur by counter-diffusion of water

vapor against air.

Adequate steaming of chips must be ensured at all times, because the buoyancy

of air entrapped in chips can critically influence chip column movement in the

digester. Other negative effects of poor air removal include pump cavitation, feed

line hammering, and inhomogeneous impregnation.

Traditionally, steaming was performed at elevated pressure in a steaming vessel

and/or near atmospheric pressure in the chip bin. Lately, it has been found that

pressurized steaming can be skipped when the duration of atmospheric steaming

is sufficiently long.

Chip Feeding

Once the air is removed from the chips, they must be brought to digester pressure,

which is done by a combination of rotary feeding devices and pumps. Liquor

is used as the transport medium for chips from the feeding system to the digester

top.