Fourdrinier wire presses

At the end of the Fourdrinier wire a couch roll removes more water from the web before it is transferred to the press section. At this point the moisture content of the web is about 80% water (20% consistency, 200g/L). The white water from the Fourdrinier is continuously recycled 1000 kg of stock at 0.5% consistency is reduced to a 25 kg mat (5 kg fibre, 20 kg water) by the time it leaves the wire and 975 kg of white water is recycled to the make-up chest and thence back to the headbox. A 500 tonne a day papermachine recycles some 100 000 m of water a day. 

The typical method of manufacturing paper involves feeding a mixture of fiber and water onto a travelling wire screen (Fourdrinier wire) so that most of the water drains through the screen and a wet web remains. The web is then subjected to vacuum and pressure (in the presses) to remove more water and receives the final drying by passing through a series of steam-heated cylinders. 

Pulp is continuously fed into the fourdrinier machine on the surface of a moving endless belt of fine mesh screening, usually made of nylon. Deckle straps prevent the liquid pulp from slopping over the sides. The screening is shaken from side to side as it moves forward to help drain the water. Suction boxes below the screening pull more water through, as a wire-mesh-covered cylinder presses on the web of pulp from above. The cylinder may be covered with a plain wire cloth to impart a wove effect, or with wire in a ladder pattern to produce a laid effect. To produce a watermark, the paper-maker attaches a wire design to the cylinder. 

Like most fabrics, paper has a right and wrong side. The bottom of the web (called the wire side) next to the screening at the wet end of the fourdrinier machine is slightly rougher than the top (or felt) side. If only one side of the p er is to be used, the smoother side is usually chosen. Paper made on a twin-wire fourdrinier machine has either two felt sides or two wire sides this is because two webs of pulp are laid down simultaneously and pressed together as the paper is dried and finished. One-sided paper is more expensive than ordinary two-sided paper. 

The Fourdrinier former is likely the most widely used forming method employed in modem papermaking. It has proven uniquely versatile in the variety of paper grades it can produce. Wire speeds up to 5000 feet per minute are possible with some lightweight grades. The following description follows the pulp flow fi-om the headbox to the couch roll just prior to the press section. A Fourdrinier wet end is shown in Figure 3. 

---