Chip Filling

The cooking cycle starts with chip filling, as wood chips are fed to the digester

with screw or belt conveyors. Screw conveyors have the advantage that their closed

design avoids the spread of wood dust which often is a nuisance with belt conveyors.

The chips drop from the conveyor into a chute and usually pass a packing device

as they enter the digester. Low-pressure steam is the most-used packing medium.

Introduced at an angle just as the chips enter the digester, the steam sets the chips

in a spiral motion and ensures their distribution across the digester cross-section.

Packing increases the amount of wood that can be charged to a digester by 10–

20%, thus leading to a higher pulp yield per digester volume. Packing also warms

up the chips and improves the homogeneity of the chip column in the digester

which is an important prerequisite for good liquor circulation and displacement

without channeling. Decent chip filling therefore is the starting point for uniform

pulp quality.

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

by the increasing partial pressure of wood moisture and by its own increasing volume.

The residual air removal must happen by counter-diffusion of water vapor

against air. During chip filling, gas is evacuated from the digester through the

strainers by means of a blower.