High vacuum dewatering

After the sheet is substantially formed and before it is pressed, it can be dewatered by vacuum application through a forming fabric. This is done with elements called high vacuum boxes or flatboxes that typically apply vacuum as high as about 15 in. Hg (about 100kpa). For many years the typical flatbox structure was of single compartment design with the width being dictated by structural considerations rather than operating efficiency. Consequently on many applications it was difficult to provide the optimum configuration of slot width and vacuum applied without jeopardizing the structural integrity of the box. Recent developments in the understanding of high vacuum dewatering has lead to widespread use of multi-compartment structures. These overcome the structural problems and allow the manipulation of dwell time at specific vacuum levels by changing slot widths and compartment sizes. However, whether single or multicompartment boxes are used, substantial opportunity for savings and operational efficiency exists from the proper application of high vacuum, particularly if at the same time attention is paid to cover material. This paper reviews the current approach to high vacuum dewatering and provides examples of existing system optimization along with new system design.