Three of the main processes in the papermachine are Forming, Pressing and Drying. Pressing involves the wet web of newly formed paper passing between pairs of rollers where the sheet is supported by felts. These felts are continuous belts, similar to a Fourdrinier wire, however they are made of absorbent felt material capable of carrying the water away from the sheet. Early machines had felts made of wool, needled into a base weave to give it strength. The development of polymer technology has replaced the wool with hard wearing synthetic materials able to withstand the stresses of modern high speed machines. The press rolls started as two plain rolls, one above the other, their objective then was to squeeze water from the sheet into the felt, thereby reducing the water content in the sheet. Press rolls, and the press nip (the point where one roll acts on the other) have all developed to suit the requirements of modern machines. The rolls now have different covers, from hard to soft, they can be blind-drilled or cross-drilled for suction rolls, also, the camber of the rolls will be calculated for each application to maintain an even pressure across the web. The Theory of Pressing shows that if the width of the nip is extended, less pressure needs to be applied at any one point but the dewatering will be more efficient. This has lead to several designs of Extended Nip Presses, also called Shoe Presses. The net result is that either the energy requirements for the paper machine can be reduced, or, which is more usual, the papermachine can run faster.
Papermaking is very energy intensive and water removal in the press section is far more cost efficient rather than using steam heated dryers in the drying section. There are many terminologies for Presses, for example, single felted, twin felted, suction press, primary press and main press. Press Felts are also called Press Fabrics and their structure can be very complex. They are available in different weights and thicknesses all designed for different positions and applications.
|
To print this FactSheet click here To return to the FactSheet home page click here
