Chip Filling and Steaming

The digester is filled with chips from a chip bin above the digester, or from a conveyor

transporting the chips from a chip silo at the woodyard. The weight of the

wet chips is determined by a conveying weigher or, alternatively, by weighing the

whole digester content by means of strain gauges. The moisture content of the

chips is measured on-line on the belt conveyers using the microwave or neutron

activation principle to record the moisture content of the whole chip volume.

The production of uniform pulp at maximum digester yield requires the maximum

weight of chips to be distributed uniformly throughout the whole digester.

Chip packers are used to obtain uniform chip distribution and greater packing

density. These devices impart a tangential motion to the falling chips and produce

a homogeneous horizontal surface of the chip pile inside the digester. The chip

packing system of choice is chip packing with steam using the Svennson system

[1]. This consists of a steam pipe with nozzles directed downward below the lower

peripheral edge of the top sleeve. The amount of dry wood charged per digester

volume unit is thereby increased by about 30–40%, depending on the steam pressure,

the moisture content, and the density of the chips. The degree of packing is

defined as the ratio of dry wood substance in a digester to the theoretical weight

of dry wood substance, assuming the total digester volume to be solid wood. The

following example should illustrate the term degree of packing (DP):

Digester volume (in m3) 300

Dry wood density (g cm–3) 0.68

Digester wood charge (in odt) 84

DP _

300 0_68 _ 100 _ 41_2_ _173_