Pre-heating
One decade ago was an optional stage where the washed chips are heated
to 40-60°C at atmospheric pressure in a surge bin (Figure 1.53b). This
softens the particles so that a better plug is formed in the conical screw
feeder (Figure 1.53c), which is a device that allows the continuous
compression (2:1 rate) of chips into the main steaming tube (Figure
1.53a), which is also known as a preheater or digester, whilst still
maintaining a high pressure in the tube. The plug screw squeezes out
some of the free water in the chips. The volume removed can be some
m³/h depending on line capacities and raw material. This water and a part
of steam condensates must be treated