Continued from Part 1 . All components mentioned in Part 1 have to be adequately prepared for optimum use in the papermaking process steps. The additives may be delivered ready for use or may have to be finally prepared according to the requirements in the mill. Fiber stock preparation includes several unit operations depending on the furnish and the purpose. Stock preparation of virgin fiber pulp needs less machinery and energy than the preparation of recovered paper which, however, is the cheaper raw mate¬rial. Fiber stock preparation ends at the paper machine chest. Here stock of high consistency is preferred to minimize carry-over of chemicals and contaminants.
Stock preparation is followed by the approach flow system connecting stock preparation with the paper machine. Its main tasks are
• to dose exactly and mix uniformly all the different components of the final sus¬pension to be delivered to the paper machine
• to supply a continuous suspension flow of constant consistency, quality and flow rate at constant pressure to the headbox of the paper machine.
The approach flow system ends at the distributor of the headbox.
The task of the paper machine is to produce paper or board of the quality re¬quired by the end user – or by the intermediate process steps such as converting or printing. The paper and board properties have to be uniform in machine direction (MD) as well as in cross machine direction (CD). Further, the paper machine has to make the best use of the quality potential of the entering stock. The paper machine includes
. • the headbox distributing the suspension across the machine width onto the wire
. • the wire section where the suspension is formed into an endless web by dewa¬tering
. • the press section pressing water out of the web by mechanical means
. • the dryer section where the residual water is evaporated • often a sizing unit where starch, or pigments are transferred onto the web
. • sometimes a coating section where coating color is applied to the web
. • the calender to finally smooth the paper or board surface.
The paper manufacturing process ends with the paper web being reeled at the reeler at full width.
By tradition and technical feasibility, coating and supercalendering for surface quality improvement have been off-line processes. Today both are increasingly integrated into the paper machine. The final activities in paper and board produc¬tion are slitting of the full width reels into smaller rolls at the winder followed by packaging the rolls for shipment.
Paper broken during the manufacturing process has to be recycled and fibers are recovered from the white water of the paper machine in a saveall. White water is fed back from the paper machine to the approach flow system and stock prepara¬tion. Fresh water is supplied to the paper machine.
Along the paper production line stock consistency varies according to the re¬quirements of the unit operations. Unfortunately the terms low, medium and high consistency relate to different consistency numbers depending on the actual unit operation (Table 1.1). It is also important to note whether the number gives the overall consistency including fibers and fillers or just fibers.