At Binet Sul Liri, we consider the felt as one of the most important, or a main vital part of the paper machine. In the past 25 years we constantly worked to improve our felt production quality and considering it as an engineering tool rather than a commodity. If it is valid for the paper and board production, the felt assumes a particular strategic value in the tissue sector.
Sheet Dewatering: Crescent Former Machines
The water removal is essentially generated by the vacuum boxes, suction rolls and traditional or shoe presses but, in all cases, the water has to be carried by the felt and through the felt itself. The paper sheet could reach the press with 12-14% of dryness and generally leave the first nip at 39-41% and 43-44% in case of a second press, actually only on extended nip presses it is possible to overcome these results and obtain up to 48-51% of dryness.
The vacuum generated by one or more suction pumps is considered correct, for high speed machines, when the specific air flow calculated at the suction pump reaches 80 lt/min/cm2 of open area. This value is going to determine the total open area in the vacuum boxes. For suction rolls, a lower specific air flow is generally utilized (25-30 l/min/cm2) in consideration of the longer dwell time (50-60 ms vs. 1-3 ms of the suction boxes). The vacuum value considered optimal is in the range of 0.3-0.6 bar, beginning from the start-up to the end of the lifetime. The felt’s permeability has to be associated to the paper machine characteristics in order to allow the correct water removal with the different paper grades and machine speed. The typical permeability value for fast machines is about 20 CFM with a start-up conditioning time that can go from a few hours
to a couple of days.